I once wrote a mail from one coworker's laptop to someone else: "I love you, please marry me"... one thing I learned from it is that something like that can and will backfire.
The next day, someone put up wedding posters about me and the other guy (I was the bride because of my long hair). The joke carried on until the upcoming summer party, at which the wedding should have taken place. However since it was the wedding organizers who chickened out, the whole thing came to an end.
Well, it was great fun while it lasted :D
Never take your coworkers' pranks seriously, or you will have a bad time