This probably isn't the coolest bug I've ever solved, but surely the one with the biggest faceplam
So I was building a Bluetooth smart watch that pairs with your Android device for the final year bachelor's project. The submission was in a 2 days and it was all ready and it suddenly stopped working.. Spent hours trying to fix it, even tried to get a replacement Bluetooth module (was out of stock -_-).. After a day's worth of freaking out I discovered that Android phones (at least the OnePlus X) don't connect to Bluetooth modules when their battery is below 15% -_- and since I was freaking out I would let the phone charge a bit and get back to debugging and it never crossed 15% so it never worked.. One day of debugging attempts later it suddenly struck me that low battery might be an issue.. And voila! It worked after charging the phone

Shouldn't such things be clearly mentioned in documentation :/

(Btw, got full for the project, got a 10/10 GPA for the semester)