Never lend a pen, even if not in the army.
At work I carry a pen to write with, and a crappy pen to loan.
I have a drawer full of pens that I don't know where they came from or how I acquired them. It wasn't on purpose. Go to someone's desk to sign something. They have a pen, it gets used. Then I walk off with it without thinking.
But some of those folks are getting wise to me. Seems like people are asking for their pens back more often these days!
Oh well, I guess there are worse reputations one could have.