If you cmd+tab out of the game, cmd+tab from/to some other windows/apps and then finally cmd+tab back into the Minecraft game window, you'll find your cmd-key being stuck.

Then try to write some text, try to delete ONE character and it always deletes the whole word. When you then just press cmd key once, it seems to be fixed and you can delete single characters again...

Same goes for using the left/right key then, jumping between words instead of characters.