Well that error is extremely ambiguous, and the only references I can see about it that some mod(s) somewhere tried to both modify the same hashmap, and under non reproducible circumstances. Pretty much without even having a mod to point a finger at, or any idea what actually caused the crash, there is not much anybody can do. You mentioned being attacked by an invisible mob, this was due to lag. During client or network lag hiccups can happen in the game that cause these sort of non reproducible bugs. I would not worry too much about it unless it becomes recurring.
