Wouldn't call it a software bug but related:

Was developing an order system to expand in the UK. We have been developing it for the last 2 years and always had a one nasty bug in the system... Whatever we do, it still appears... Tried debugging to find the source, tried covering with tests - nothing helped it was still there. We even rewrote the whole system 3 times and it still was there!

One day, we have been given a stupid request from our manager - take a black background and make it even more blacker... That was it and I went to the CEO with letter where I stated that we should remove the manager... As I'm the Senior there, he did ask me why and eventually removed the manager...

Oh my guys, I've never felt so good after removing a bug! Since then - our application went live, we had our first customers and we were happily rolling new updates. And the best part - there was no BUG! Everything we did just had undocumented features or missing links but we haven't really had a single bug that was not caught by our automated tests!

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Moral of the story:
Not only software can have bugs. People also can be "bugs" while bugging you about every single details they think is not working correctly.