I didn't forget something I had packed, but I did get briefly held up by TSA in Atlanta on a return from Afghanistan one time.
Turned out a 9mm casing had ejected into the pen holder stitched to an outside pocket on my pants leg. Not sure if it was detected by metal detector or chemical sniffer, as it was initially noted when I went through that new machine that rotates around and blows air at you.
The initial TSA person was flipping out over a piece of expended brass in a tube pocket that I never use. The TSA supervisor who responded thought it was kind of funny, and sent me on my way.