happened to me when I spawned mob-only item that doesn't support player (had to add option for players to use it in the code) or when I have added new item recipe but nulled some stuff accidently

if you have got the same issue then
without code messing the /killme or recovering recent backup should do the job


in both cases my player.log told me the issue so it was easy to know what happened


edit:
I didn't think of it... but the guy after me might be right
Some sort of other controller/program interfering with game is more likely than my messing around