This old guy was hanging around the counter leafing through a big New York Times Page One book, the kind that reprint the first page of the paper over the course of years- in this case, 1851-2002. He called my attention to the 1945 crash of a B-52 into the Empire State Building.
"Y'know, it seems funny, a plane crashed into the Empire State Building and it doesn't fall down, but yet on 9/11 those buildings fell down."
"Yeah, well, there's a huge difference between a prop-driven WWII aircraft and a modern jetliner with full fuel tanks." I replied, breaking my own rule of not engaging crazies.
"Uh, I guess we'll just never know what happened on 9/11," he muttered.
"It's pretty obvious what happened, terrorists hijacked a bunch of planes and crashed them into buildings."
This convinced him I wasn't a receptive audience for his BS and he wandered off.