Why does a bug I fixed keep coming back?

Either your fix never actually worked (it addressed the wrong cause or was incomplete, so the bug was always there, just seemingly gone), or a later change reintroduced it (a regression that undid the fix). Verifying the fix in the field, by watching whether the issue stopped occurring on the fixed version, catches the first case immediately, and per-version tracking catches the second by showing the bug reappearing on a later build. Without verification, a non-working fix sits in a false 'fixed' state until the bug 'returns.'

How do I make sure a bug fix actually sticks?

Verify it and watch per version. After fixing, confirm the issue actually stopped occurring in the field on the fixed version, rather than assuming the fix worked, this catches a fix that addressed the wrong cause or was incomplete. Then track per version so if a later change reintroduces the bug (a regression), it's visible as the issue reappearing on a newer build, tied to that release. Verification (field, per version) catches both the non-working fix and the regression, so you know a fix truly stuck.

What Causes Bugs to Reappear After Fixing?

Q: What causes bugs to reappear after fixing?

Two main reasons: the fix didn't actually work (it addressed the wrong cause or only part of the problem, so the bug was never really fixed and only seemed to be), or the bug was reintroduced by a later change (a regression where a subsequent change undid the fix or re-broke the same thing). Merge/integration issues losing the fix, a fix that didn't ship, or a different bug with the same symptom can also be behind it. The two categories, fix never worked versus fix undone, need different responses.

Quick answer: Bugs reappear after fixing because the fix didn't actually work (wrong cause or partial fix), or the bug was reintroduced by a later change (a regression that undid the fix). Verifying fixes and tracking per version catches both.

Few things are more frustrating than a bug you thought you fixed coming back. It has two distinct causes, and telling them apart matters. Here's what causes bugs to reappear after fixing.

The Two Reasons Bugs Come Back

A bug reappearing after a 'fix' means either the fix never worked or something undid it. The specific causes:

The fix didn't actually work, it addressed the wrong cause, so the bug was never really fixed
An incomplete fix, it fixed part of the problem but not all of it, so the bug persists in some cases
A later change reintroduced it, a regression where a subsequent change undid the fix or re-broke the same thing
A merge or integration issue, the fix lost or reverted during merging or integration
A different cause with the same symptom, a new bug producing the same visible problem, looking like the old one returned
A fix that didn't ship, the fix not actually included in the build players have

The two main categories are 'the fix never worked' (wrong/incomplete cause) and 'the fix was undone' (a regression), which need different responses.

Why Verification Catches the First

If a fix didn't actually work, you'd know immediately if you verified it, by watching whether the issue stopped occurring in the field on the fixed version. Skipping verification lets a non-working fix sit in a false 'fixed' state until the bug 'reappears' (though it never left).

Bugnet shows whether an issue continues after the fixing version, so you can confirm a fix worked rather than assuming. Verifying fixes catches the 'fix never worked' case immediately, before you believe a bug is gone when it isn't.

Why Per-Version Tracking Catches the Second

If a later change reintroduced the bug, per-version tracking catches it: the issue appears again on a newer version, flagging that a recent change re-broke it (a regression). Knowing which version it returned on points at what change brought it back.

Bugnet tracks issues per version, so a bug reappearing on a later build is visible as a regression tied to that release. So bugs reappear because the fix didn't work or a later change reintroduced it, and verifying fixes (field, per version) catches both, the non-working fix immediately, and the regression when it returns.

Bugs reappear because the fix never worked (wrong/incomplete cause) or a later change reintroduced it (a regression). Verify fixes in the field to catch the first, and track per version to catch the second.