Not every bug is worth fixing, and a team that hasn’t decided which ones aren’t will fix the wrong ones.
The backlog fills with defects nobody hits. Then a real one lands and there’s no room left to work.
Sorting that out starts with knowing what kind of defect you’re holding, because the type decides who fixes it and how urgently.
The types of software bugs below are the six a working team meets, and each one fails in a different place.
What This Article Covers
- Classes: six of them, each breaking a different part of the product.
- Judgements: severity and priority get set by two different people.
- Patterns: a repeating bug is a phase problem, not a code problem.
What Counts as a Software Bug
A bug is any gap between what the software does and what it was specified to do.
That definition matters because it puts missing requirements inside the count. A feature that works exactly as written, and the written version was wrong, is still a defect.
Teams that only count crashes undercount their real quality position by a wide margin.
Six Bug Types and What Each One Breaks
The classes below differ by where they surface, and that’s what makes them useful. A team can route each class to the person able to resolve it.
Functional Bugs
Functional bugs break what the software is supposed to do.
The save button doesn’t save. The filter returns the wrong rows. These are the easiest to describe and the easiest to test for, which is why they’re the ones most teams catch.
Logic Errors
Logic errors produce the wrong answer from working code.
Nothing crashes. A discount applies twice, a date rolls over at the wrong hour, a total rounds in the wrong direction. They survive testing because the software looks fine while doing the wrong arithmetic.
Integration Bugs
Integration bugs appear only where two systems meet.
Each side behaves correctly alone. Together, one sends a field the other truncates, or retries a request the other already processed. They’re rarely reproducible in a single environment, which is what makes them expensive to chase.
Performance Bugs
Performance bugs are correct behavior arriving too late to be useful.
A report that takes four minutes is not wrong, it’s abandoned. These surface under real data volumes and almost never in a test dataset.
Security Defects
Security defects let someone do what they shouldn’t be able to do.
The software works as built, and what it was built to allow is too much. An endpoint answers without checking who’s asking, or a record is visible to a role that should never see it.
They’re the class where the reporter is often outside your company.
Data Bugs
Data bugs corrupt what’s stored rather than what’s shown.
These are the worst to discover late, because fixing the code doesn’t fix the records already written. Every day the defect runs adds rows to the cleanup.
How Teams Decide What to Fix First
Two separate judgements decide it, and confusing them is the common failure.
Severity describes how badly the software is broken, and whoever found it sets that.
Priority describes how soon it gets fixed, and whoever owns the roadmap sets that.
| Situation | Severity | Priority |
|---|---|---|
| Checkout fails for every customer | High | High |
| Data corrupts quietly in a rare path | High | High |
| Company name misspelled on the signup page | Low | High |
| Crash in a feature two customers use | High | Low |
| Slow report an analyst runs monthly | Low | Low |
The third row is the one teams argue about, and it’s the one that shows why a single ranking number never works.
How Bug Types Trace Back to a Phase
Recurring defects of one class point at a phase, not at a developer.
An authorization bug that keeps returning was decided at design time, so patching each instance leaves the source intact. The fix belongs in the threat model that phase produces.
That’s the argument for treating quality as something the secure development life cycle owns at every stage rather than something testing catches at the end.
Track the phase each defect was born in, not the sprint it was found in. The two are rarely the same, and only one of them tells you what to change.
Software Bug Questions Teams Ask
What are the main types of software bugs?
The main types of software bugs are functional, logic, integration, performance, security and data defects. Functional and logic bugs break what the software does. Integration and performance bugs break how it behaves with everything else.
What is the difference between severity and priority?
The difference is who decides. Severity describes how badly the software is broken and is set by whoever found it. Priority describes how soon it gets fixed, and a cosmetic bug on a signup page can be low severity and top priority.
Which bugs are the most expensive to fix?
Defects born in requirements and design cost the most, because everything built afterwards assumes they were right. A wrong assumption about who may see a record spreads into every screen that reads it. A misspelled button is contained to one screen.
Is every bug worth fixing?
No, and pretending otherwise is how backlogs stop being useful. Teams that fix everything spend their capacity on defects nobody hits, then run out of room when a real one lands. Close the ones you have decided not to fix, rather than leaving them open forever.
How do teams stop the same bug class coming back?
To stop a class returning, teams trace the defect to the phase that created it and change that phase rather than the code. A recurring authorization bug is a design problem, so the fix belongs in the threat model. Patching each instance leaves the source intact.
Close the Ones You Won’t Fix
Open your bug backlog and read the oldest twenty.
Most teams find defects that have been open long enough to answer the question themselves: nobody hit them, nobody escalated them, nobody will.
Closing those is not giving up on quality. It’s what makes the remaining list mean something.
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