I had 2 flame arrows in my inventory when somebody bartered to give me 30 bomb arrows that cast fireball on hit.
When the transaction was completed, however, my 2 flame arrows become 32 and the 30 bomb arrows were lost.
Poor Henry Harrison is out 300 gp :(
Yes, this has been happening for a while now. It's especially annoying when you buy these from a certain merchant in a random area for a very tidy sum of gold, and they all combine.