I'm not sure this is the only case where this happens, but I noticed that if I'm aiming at something that dies during aiming, and attempt to snap fire at a new target instead of re-aiming, I get this:

That weapon must be in your right hand to fire!

This is the only sling I have, and I can immediately FIRE again and it works just fine. Weapon never moved from my right hand, was not unloaded, didn't stop aiming or start aiming at the new mob.

Correct. This actually happens with any aimable ranged weapon, as I've learned. Aim, kill with tm, fire

It should auto-face a new monster, but if you fire before you get the message that you stop aiming because it's dead, you get that weird message about the weapon needing to be in the right hand.