While coding in C, I once forgot to add a semicolon at the end of a while loop polling a register value.
The logic required me to make it zero as soon as it read non-zero and continue the rest of the process. Hence the 'while' that missed the semicolon ended up being a single instruction assignment to the same volatile register that I kept polling. This caused synchronisation issue with the FPGA, and my code got stuck in an uncertain infinite loop.
Took me 2 days and a silly, yet valid question from my teammate to figure out the cause of this stupid bug.