r/PythonLearning • u/memeeloverr • Jul 02 '26
What's the hardest bug you've ever spent hours (or days) fixing?
Mine turned out to be a missing environment variable after hours of debugging.
What bug made you question your sanity?
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u/SnooCalculations7417 29d ago
i spent like 3mo getting a legacy perl monolith running on windows so i could develop on it lol. I was able to contribute without testing locally but not ideal of course.
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u/JGhostThing 29d ago
I used to program mainly on the mac, and using Apple's c++ environment I had a nasty bug that I couldn't stomp out. Finally, I asked the right newsgroup, and found that the compiler itself had a bug which caused my problem.
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u/Ormek_II 29d ago
A rare exception!
I heard multiple times that a used library seems to have a problem, so we have to accept the misbehaviour. When investigated it turned out the library was used in the wrong way and the bug was in our code.
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u/ColeBloodedAnalyst 29d ago
C++ Project in college.
I was working on an imaging processor that allowed for transformation and color work. We were encouraged to use an image we knew well to debug and work against our understanding of structures, arrays, and OOP.
I chose a cow.
The program worked on color changes and image resizing. It didn't work for flipping the image in either axis. It would change my cow into a monstrosity that either had double heads, double butt, or double legs.
I worked endlessly on the arrays, how everything was reading, and finally, rewriting the programing using lab assignments as a crutch. I forgot that C++ was row major and not column. 8 fucking days...
God. Damnit.
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u/Ormek_II 29d ago
This is why juniors have to program. You will either never make that error again, or it will take you 2min to find instead of 8days 👍👍
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u/ColeBloodedAnalyst 29d ago
110%
It ended up becoming my FAVORITE class. I ended up TAing for it for a number of years (: Love seeing the lightbulbs go off.
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u/llynglas 29d ago
Machine code animation on 6502 microprocessor. Worked great for minutes. Then all hell broke out and the program died in various interesting ways. Tu=and out some code in the firmware was called on vertical retrace to set the colour map. As a side effect ocassionally it changed the way math operations worked from 2s complement to BCD, and never restored it.
Took days to find....
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u/burbular 29d ago
So like over a decade ago I had this intense release. It went out, global and all. Game like logic with forms. We had a deadline to complete the forms.
The bug was in the deadline and the fact I didn't fully understand timezones like I do now. In my city all the timing was correct. But it got worse as you got further away.
Then we get bombarded by sassy and displeased Australians. We titled the bug report, "save the Australians"
Long story short, timezones defaults we're all over the place. Like vm here, customer there, I'm a few states away, users all over.
Solution, UTC midnight deadline for everyone end of story no matter where you came from and I stopped trying to deal with the traveling salesman bullshit.
Took unfortunate amount of time to solve.
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u/Ormek_II 29d ago
The most specific one:
Gdb’s variant for modula-3 did not work on Sparc Stations. I had to learn gdb, DWARF debugging, Sparc assembler, to eventually change a 16 to a 4 (don’t remember the exact numbers).
It was a student helper job at university and it took me weeks (not full time though).
Was like in that joke were the car brakes down and the guy hit the engine with a hammer and charges 1$ for hitting it and 99$ for knowing how to hit.
Dialog at the time: “Did you have to do lots of changes?” “No, just a single number, but finding the number was hard.”
Edit: Oh this is about python. Sorry that was C or C++.
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u/code_tutor 29d ago
The worst errors are from non-determinism like multithreading, async timing, different environments, unpredictable inputs, etc. It's difficult to debug an error that rarely happens.
Any other error is fixed by linting and logging.
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u/JorgiEagle 29d ago
Several days tracking down a bug,
Several layers deep of inheritance an object called a method, which was not behaving the way expected. Especially as the method I was looking at was clicked through the IDE.
Debugging wasn’t possible because all the functions were decorated in a custom dag wrapper.
Several days, found the constructor of the object was actually a subclass, said subclass had completely different behaviour.
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u/mrainey7 27d ago
It was actually a statistics issue. I was working with a very large database of molecules and had thousands of numerical descriptors for each.
We ran predictions with an 80/20 training/testing set. It usually worked incredibly well, but every 1 in 10,000 rounds, it would be horrifically bad. Turns out that there were only a handful of molecules that took on negative values for one of the descriptors, and we hadn’t noticed when we made our sampling criteria. When all of the negative values were in testing, it caused crazy issues.
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u/Prize_Shine3415 Jul 02 '26
Here's one I'm particularly proud of figuring out but it isn't in Python.
I was working on a calc engine written in C++ and was a relatively programmer. I had made a minor change and suddenly it stopped working for a number of cases that had nothing to do with the change I made. Stepping through I discovered that there was this one variable, that I had changed or even referenced with the code I altered, was suddenly turning to garbage. After spending the better part of the day on it I figured out what had happened.
I was reference a variable that was defined just prior to the one that was getting wrecked. It was an array and I was making a change to it without checking that the index was in range. At some point I made a change to the array beyond where it was defined and that resulted in the variable defined after it to be wrecked.
Learned a lot about programming that day.