After interacting with a mob, you can't use items while looking at a mob

Complicated sounding title, but really simple to reproduce.

1. Get some snowballs and a cow
2. Look directly at the cow up-close and try to throw a snowball (it works fine)
3. Get a saddle and a pig
4. Use the saddle on the pig, but don't ride it
5. Try to use the snowballs on the cow again
   →  You can't anymore

Not perfectly consistent, try relogging until you can reproduce.