What's the bug that took you very long to find but made you smile when you found the problem?

Working on a big website, the designer told me that the font of overlays is blurry and we should find a solution. After hours of searching, I just didn't care anymore. Only a week later by accident I found out in a stackoverflow thread that in CSS translate3d is calculating floating point pixels while translate calculates integer pixels. So I changed everything to translate and all was fine. 😉
I hate frontend.

2 years (!&@). Had an issue that occured only on production, randomly.. Found so many issues and none fixed it. Eventually i found it, was an issue with the error handlers logic. God damn

Missed a semi colon..

😬