it happens frequently but if you relog it fixes itself.

when you turn on hitboxs it thinks that you are still holding f3 so it makes it so all the keys that do things with f3 are unusable. it probably won't happen first time you try to do it.

if it helps i was on hypixel.

update if you hit some keys it freezes the game until you let go of it

however there is a work around you have to hit a key like CTRL to get f3 to work but it does not have to be CTRL which is the one that can mess up the keys shift works too and so far has not done it yet.