First time would be when I had two teachers working of a bug that resultet in the CSS not loading / updating properly. It took me around 1 hour as well to realize that the text file was encoded with some obscure windows encoding, that doesn't load chrome (and maybe other browsers) can't load properly.

Thanks notepad++, will never use you again for web development.

Second time would be when I presented an app I made too the same teachers in order to boost my mark. Bugs that never appeared before every where and the basic functionality of the app was questionable. I hadn't worked on that app for half a year and had to fix as much as possible in a short amount of time, which might have made it worse.

Don't ask why but thanks to my knowledge about the france language I got a better mark, even though it was spanish