We wrote a fake Dear John to break a little greeny's heart.
I was on splits with the District Leader. His greenie companion was obsessive over his girlfriend he left back home and talked about her all the time. During our lunch break, we saw that there was a letter from her in the mail. We pulled it out of the envelope and hid it. The DL somehow was able to identically replicate her handwriting (an amazing talent in retrospect) and wrote a pretty detailed letter about how she had fallen in love and was engaged to another guy. On the back, we scribbled a supposed message from the new boy friend. It said something like "Thanks for keeping her warm for me, Sucker." We put the letter back in the envelope from her, hid the real letter, and put it back in the mailbox.
Needless to say, he was a broken man. Completely shattered. We let it simmer for about a half hour before revealing the hoax. Looking back, probably not a great idea. And also a pretty big waste of time. But a great memory.