When you're in creative mode, and you try to put a stacked item into the item list it just appears back.
only solution is to singlehandedly put them in the item list (only tested with signs).

Created a better fitting ticket to handle this: #67

More info after testing:

While in creative mode, placing a large stack (2 signs, 17 snowballs) into the Item Selection screen will cause them to drop onto the ground.

Placing a large stack outside the Item Selection screen will cause a duplicate to also drop onto the ground. (2 signs become 2 stacks of 2 signs).

For now, I've just made it ignore creative in order to prevent the strange behavior. I'll try to get a new version up tonight after some more testing.

I'll take another look at things to handle and fix in creative in the future, but it's not a huge priority for now as it shouldn't break anything. Let me know if there are any other issues that come up with creative though.

Can't do much about this until bukkit handles the current creative window(s).