When the List Comparer seems to misbehave, the cause is usually one of two things: something stopped the tool from running, or it ran but the result is not usable. This guide starts with the fastest checks, then walks through the common problems and exact fixes.
Quick Checks First
- Refresh the page - a stalled session is the most common cause of nothing happening.
- Confirm the input is not empty and actually contains what you meant to submit.
- Try the smallest possible input; if that works, the issue is size, not the tool.
- Turn off browser extensions that block scripts, then reload and test again.
- Update or switch browsers if the problem repeats across sessions.
Common Problems and How to Fix Them
Everything shows as unique.
Both lists must go in their own box - one pasted list cannot be compared against itself.
Lines that look the same did not match.
Matching is exact - check for stray spaces, punctuation and capitalisation before comparing again.
The result is missing a line I can see.
An empty line counts as an entry - remove blank rows so the sections show only your real items.
I want case insensitive matching.
Normalise both lists first by lowercasing a copy, then run the comparison on the tidy versions.
The output is too long to read.
Copy each section separately - common, only in A and only in B are listed in their own blocks.
I compared the wrong versions.
Paste fresh copies of both lists and rerun - each comparison uses only what is in the boxes at that moment.
When It Works but the Output Is Poor
If the tool runs fine but the result misses, the problem is almost always the brief, not the software:
- Pasting both lists into one box
- Expecting similar lines to match
- Comparing lists with different spacing
- Ignoring case differences
How to Stop It Happening Next Time
- Keep a saved prompt template for every task you repeat.
- Change one setting per generation so causes stay obvious.
- Run a two-minute sanity test with known input after any update.
- Note every failure and its fix; a short troubleshooting log pays for itself.
Still Stuck?
Reproduce the problem with the smallest possible input and note the exact steps that triggered it. A one-line description of what you did, what you expected and what happened is enough to get a precise answer from support or a colleague.
Frequently Asked Questions
How are lines matched?
Each line is compared exactly against the other list, so wording and spacing must be identical to count as a match.
Can it handle long lists?
Yes - paste as many lines as you need into each box and the sections update when you run the comparison.
Does it change my lists?
No - both lists stay as pasted and the results are reported alongside them.
Are my lists uploaded?
No - the comparison runs in your browser and nothing is stored.
Related Guides
- How to Use List Comparer
- What Is a List Comparer and How Does It Work?
- List Comparer Examples: Real Scenarios and Results
- List Comparer for Teachers, Planners and Developers
- Best List Comparer Alternatives Worth Trying
- List Comparer Tips and Best Practices
- Is List Comparer Safe? Free vs Paid
- More list tools and guides