See video
https://www.youtube.com/watch?v=9eDSBS2Hu4I&feature=youtu.be

I've been getting this since the 0.49 push to stable. When spamming LMB to hit zombies (live or dead) my weapon will often switch to the 3rd item in my hotbar. At least I noticed today it always seems to be the third item in my hotbar.

I've had this with the fire-extinguisher, fire-axe, the baseball bat and the cattle-prod. So it seems to be a general melee issue.

It doesn't seem to happen when hitting floor, walls or ceiling, but only zombies and maybe players or their bodies.

Interestingly, at one point you can see my character "stab" using the canteen. Something which is generally also not possible.