Trade Sign Bug

In the last past days i'm just finding too much bugs :O

Minecraft Username: Kasert

Brief Description: I was spamming buying a trade sign until my inventory was full, i right clicked one more time and i lose not only what the trade sign gived to me but i losed my money too

Instructions: 1.Pick any random selling sign
2.Full your inventory
3.Right click sign

How many times did you recreate this bug?: they first time this bug appeared to me when i was spamming buy dirt at /warp eccheadmuseum

Result:You will not only lose the dirt from the sign but money too and the owner will get your money just like a donation sign (Nothing for 5$)

Expected Result: Don't know if this works with other signs outside other world like stations

Evidence: (1 Stack of dirt = 5$ the owner of the sign received more than the all of this dirt costed without giving me nothing)

Hope you get it what I wrote ! (This is a trade sign bug can't explain with a title :U