when testing my friend and my own custom maps again we found out this cube bug :p
no idea if it only applies for custom made maps
as i cant find any original level to reproduce this
but take a look at the vid ^^ http://www.xfire.com/video/466305/
already reported on the portal2 forum ofc this is cheating

It's a known bug.

I'm not sure is a game bug.. For example, in Damagepy's map you can stack 2 lasercubes, and if you remove the lower one, the bug occur.. but in the first room of chapter4, you can do the same thing, but the bug won't happen...

that's 100%badass is what it is lol
i didn't know about the glitch though... So thanks for that sambo