I've been trading my roasted meat for arrows for portability and packing them on my reindeer stag. I noticed that if I just packed new arrows on the stag, they weren't automatically stacking so I've been forced to unpack, drop, and repack them.
I just ended up with 259 arrows and while packing them got the error message "Burden of 259 arrows is too much for the reindeer stag to carry!" so I decided to see what I could unpack since every slot is used up and found 255 arrows on the stag. I unpacked them, dropped them with the others and attempted to repack them. After playing around with it a few times, I've now got over 1000 extra arrows I have to discard.
Edit: I'm running Version 3.50 (stable)