I was doing a rental checkout in a G-1000 172 one afternoon out underneath the DFW class B. I was in the process of explaining to the guy in the left seat that the TIS display will only pick up planes with a working transponder, and that it's no substitute for looking out the window. Not ten seconds after that I zero-G'd the thing to avoid some nutjob in what appeared to be a partially completed homebuilt crossing just in front of us. He was close enough that I could see his curly blonde hair waving in the wind under his david clarks... (It was doing this because the damn thing didn't have a canopy on it yet!). I would have gotten his N number, except it didn't have one of those either... You never know who you're going to run into out there... (Though, hopefully not literally)