One guy came in and without introducing himself started to write numbers on the whiteboard. He drew borders around some of them. Then he sat down, introduced himself, and said that what he wrote on the whiteboard was machine code. After explaining some of his nomenclature he asked me what I think that code was doing.
Another guy asked me to solve a problem (don't want to say what specifically it was, because they may still use that question)... Anyway, I could not for the heck of me solve that problem. After 45 frustrating minutes of this he started to write down the solution, then hesitated, then stopped, saying something like "Hmm... It's not quite as easy as I thought".
In the end I ended up declining their offer. I spent some time on their campus, and the vibe just did not feel right. Can't put my finger on it, though.