I first stumbled into this glitch when wandering home with some cows. I was actually trying to dig through a mountain, and had a single wheat in my off-hand (which was my left, in case it matters), and switched between a pickaxe and torches in my dominant hand. Well, I noticed the number on the stack appeared with a red zero, and had no clue why that happened. I think I right-clicked the air and it disappeared (I don't remember, but I might have put it in my right hand when I right-clicked).

I next found this glitch when feeding dogs with some raw steak. Again, I had the food in my off-hand, and when I used up the food, it stayed in my hand, and had a red zero. I fed another dog, and it went to -1. Then, I tried to eat it, and it went to -2. I thought I might be able to get infinite cooked steak from this glitch, so I put the negative stack in my furnace, and it produced a steak, but then disappeared. The code must check for items "<= 0" instead of "== 0", which makes sense in light of this glitch.

Furthermore, if I have a negative stack, and another, single beef, I can pick up the negative stack with the mouse, then right-click the singly beef until it reaches 64. The game can't tell that you shouldn't be able to do that, so it decreases the negative stack by 1, and increases the positive stack by 1. Note: if you have more "negative steak" than normal steak in your inventory, no matter how many stacks of normal steak you have, if you double click any steak, it consolidates, just as you might expect, into a single, negative stack.

Yet another odd property of a negative stack is that if you throw it onto the ground and pick it up, you get the negative stack back, but it leaves an unobtainable item on the ground where the negative stack once was. I think it must be another zero-stack.

This makes me wonder what will happen if I use a hoe in the left hand...