When you see a print screen of hammer and start trying to use it >_<
This is more Portal 2 than hammer, but today at work (I work in a coffee shop) I wanted to know what size coffee the customer was ordering, so I mentally tried to press TAB to see what my co-worker (coop partner) was seeing. Needless to say it didn't work... I don't have a TAB button with me at work =\
You're in the cinema, the curtains go open and you think; those curtains are probably parented to a func_door.
You know you've been using Hammer too much when all Valentines day means to you is, "I should make some tests in Hammer with a Companion Cube meme in it". I never got around to it, unfortunately.