Not sure if anyone else has come across this before, but it cost me quite a few fletched shafts today.

I had a bundle of 30 or 40 mostly finished arrows in my hand, and gathered 10 more I had on the floor. The bundle counted as a single unit, so the new bundle count was only 11 instead of 60.

Had to scroll back through my history to verify it. After I spotted what happened, I recreated it a couple more times, just to make sure I wasn't confused.