Probably not worst but back when I was in graduate school I used '=' instead of '.equals()' for a string, by mistake. My professor was so pissed and taunted in front of whole class about how dumb people are here without taking any names(btw I didn't realize that guy was me until later) and gave me a low score

I told her later it really doesn't matter if I used either because '=' worked just fine in my case. She was a little more pissed after knowing that I wasn't wrong. 😜