I'm fairly new to this, so I'm sure that most of my dumb mistakes have yet to be made. On my third or fourth find or so, I opened the cache and was pondering what to take in trade. I signed the log with the pen I brought, then while going through the swag I found a really nice little ink pen that I traded for. It wasn't until a few finds later that I realized the pen I took was most likely left there for people to sign the log with, and not part of the cache!