The 5 Most Common Medical Billing Errors (And How to Spot Them)

Why Checking Your Medical Bill Matters

Medical billing errors are far more common than most patients realize. According to a widely cited analysis, roughly 80% of medical bills contain at least one error Source. The financial impact is staggering - the U.S. healthcare system loses an estimated $935 million per day to billing mistakes and fraud Source. For individual patients, a single billing error can mean hundreds or even thousands of dollars in unnecessary charges.

As we detailed in our complete whitepaper on medical billing errors, these mistakes aren’t always intentional - many result from the sheer complexity of medical coding systems with over 10,000 CPT codes and 70,000+ ICD-10 diagnosis codes. For a deeper dive into what these codes mean and how they work, see our plain-language guide to medical billing codes. Whether intentional or accidental, the result is the same: you pay more than you should.

Understanding the most common types of errors is your first line of defense. Below, we break down the five billing mistakes that account for the majority of overcharges - and show you exactly how to spot each one.

Error 1: Duplicate Charges

What it looks like: The same service, procedure, or supply appears more than once on your itemized bill for a single date of service. For example, you might see CPT 99213 (office visit, established patient, moderate complexity) listed twice on the same day when you only had one appointment.

Duplicate charges are the single most frequent billing error, accounting for roughly one in four billing mistakes Source. They often occur when a charge is entered manually and then also auto-populated by the electronic health record (EHR) system, or when a claim is resubmitted after a processing delay without the original being voided. For a step-by-step walkthrough of exactly how duplicates appear on bills and what to do about them, see our complete guide to spotting and disputing duplicate charges.

How to spot it: Review your itemized bill line by line. Look for identical CPT codes, identical dollar amounts, and identical dates of service. Pay special attention to ancillary charges like lab work, imaging, and supplies - these are the most commonly duplicated items.

How NilesAI catches it: NilesAI’s duplicate detection engine automatically scans every line item on your bill, cross-referencing CPT codes, dates of service, and charge amounts. When it finds identical entries, it flags them for review and calculates the potential overcharge. Learn more about how this works in our scan engines overview.

Error 2: Upcoding

What it looks like: Your provider bills a higher-complexity (and higher-cost) CPT code than the service you actually received. For example, you visit your doctor for a routine follow-up and receive a bill coded as CPT 99215 (high-complexity visit, ~$250) instead of CPT 99213 (moderate-complexity visit, ~$150).

Upcoding accounts for approximately 20% of billing errors and is a particular focus of the Office of Inspector General (OIG) at the Department of Health and Human Services. In 2023, the OIG recovered over $2.1 billion in healthcare fraud judgments and settlements, with upcoding among the most common violations Source. The American Medical Association (AMA) maintains the CPT code set and publishes guidelines that define what level of service each code represents.

How to spot it: Compare the CPT code on your bill to your medical records. Key questions to ask:

• How long did the visit actually last?
• How many body systems did the provider examine?
• Was the medical decision-making straightforward or complex?
• Does the code level match what you experienced?

You can look up CPT code descriptions on the AMA CPT website to understand what each level entails.

Error 3: Unbundling (NCCI Violations)

What it looks like: Services that should be billed together under a single complete code are instead billed as separate procedures at higher individual rates. For example, a complete blood count (CBC) panel might be billed as individual tests - white blood cell count, red blood cell count, hemoglobin, hematocrit - each at its own price, rather than as the bundled CBC panel code.

The CMS National Correct Coding Initiative (NCCI) maintains edit tables that define which procedure codes should be bundled together. When a provider bills component codes separately in violation of these edits, patients and insurers pay significantly more. Unbundling violations represent roughly 18% of all billing errors Source.

How to spot it: Look for multiple related procedure codes on the same date of service, especially in lab work and surgical procedures. If you see several line items that seem to describe parts of a single service, unbundling may be at play. Cross-reference the codes against the NCCI edit tables, which are publicly available.

How NilesAI catches it: NilesAI’s NCCI compliance engine checks every combination of CPT codes on your bill against the current NCCI edit tables, flagging any pair that violates bundling rules. This analysis would take a human coder hours - NilesAI completes it in seconds.

Error 4: Incorrect Modifiers

What it looks like: CPT modifiers are two-digit codes appended to a procedure code to provide additional context - such as which side of the body was treated (modifier -LT for left, -RT for right) or whether a procedure was performed by a different provider during the same session. An incorrect modifier can change how a claim is processed and what you owe.

Common modifier errors include applying modifier -25 (significant, separately identifiable E/M service) without documentation to support a separate evaluation, or using modifier -59 (distinct procedural service) to bypass NCCI edits inappropriately. The AMA publishes detailed modifier guidelines, and Medicare has specific rules about when each modifier is valid.

How to spot it: Check whether any modifiers on your bill make clinical sense for the services you received. If you had a procedure on your right knee but the bill shows modifier -LT (left side), that is a clear error. Similarly, if modifier -59 appears on a code that should be bundled with another service, it may have been added to circumvent bundling rules.

Error 5: Balance Billing and Fee Schedule Overages

What it looks like: Your provider bills you for the difference between their full charge and what your insurance allowed - a practice known as “balance billing.” Under the No Surprises Act, which took effect in January 2022, balance billing is prohibited in many circumstances, particularly for emergency services and care received at in-network facilities from out-of-network providers.

Despite these protections, balance billing violations persist. A 2023 survey found that 1 in 5 emergency room visits still resulted in a surprise bill Source. Additionally, providers sometimes bill charges that exceed the contracted fee schedule with your insurer - meaning even in-network charges can be inflated beyond what should be allowed.

How to spot it: Compare the “billed amount” on your Explanation of Benefits (EOB) to the “allowed amount.” If you are being asked to pay more than the patient responsibility shown on your EOB, you may be experiencing balance billing. For emergency services and situations covered by the No Surprises Act, you should not be billed beyond your in-network cost-sharing amount. Emergency department bills are particularly prone to balance billing - our detailed breakdown of ER bill anatomy explains exactly which line items to scrutinize.

What to Do If You Find an Error

Finding an error on your bill is only the first step. Here is what to do next:

1. Request an itemized bill if you do not already have one - every patient has the right to one under federal law.
2. Request your medical records for the date of service in question to cross-reference against the billed codes.
3. Contact the provider’s billing department - many errors are corrected with a single phone call. Reference the specific CPT code and date of service.
4. File an appeal with your insurer if the error affected your insurance claim.
5. File a complaint with your state insurance commissioner or the CMS No Surprises Help Desk if your dispute is not resolved.

For a detailed walkthrough, read our complete guide: How to Review Your Medical Bill Step by Step. Not sure what a billing code means? Check our medical billing glossary. Need to compare charges? Use our cost lookup tool to see Medicare rates for over 200 procedures. Have more questions? See our FAQ.

How NilesAI Automates Error Detection

Manually reviewing a medical bill against NCCI tables, fee schedules, and EOB documents is time-consuming and requires specialized knowledge. NilesAI was built to handle this complexity automatically.

When you upload your itemized bill and EOB to NilesAI, our suite of scan engines runs every check described in this article - duplicate detection, upcoding analysis, NCCI compliance, modifier validation, and fee schedule comparison - in seconds rather than hours. Each flagged issue includes a plain-language explanation, the specific codes involved, and an estimate of the potential overcharge.

Whether you are a patient trying to understand a confusing bill, an attorney reviewing medical liens in a personal injury case, or a billing advocate helping clients work through the system, NilesAI gives you the tools to catch errors before they become financial burdens. Not sure where to begin? Our bill diagnostic tool walks you through the most common error types and points you to the right resources based on your specific bill.

Ready to check your bill? Upload your documents to NilesAI and get your analysis in minutes.

You can scan a bill for free now to see what NilesAI finds.