User blog comment:Christian Wilson/Inv. Stacking glitch/@comment-4583259-20111125171200/@comment-4583259-20111125202717

Yes, that's right. It adds the number in one stack to both stacks, so after the quantities get to be roughly equal the effect is to double both stacks simultaneously. Repeating my numbers again, but adding a few more to make it obvious:

To get those numbers, I started with 255 and 1, and each time I put the smaller stack on top of the bigger stack. (If you do it the other way around, and put the 255 on top of the 1, then I think you get 255 and 256, and then they'd both be almost equal from that point on)

One number is a power of 2, and the other number is 255 plus the same power of 2. After the power of 2 gets big enough, both stacks are close enough so you can think of them as being the same.