This happened a few years ago. My wife is pretty crafty, always making picture montages and stuff like that. One day she made a pretty cool looking shadow box, and inside of it says something like "the best dates in our lives", and has four dates. The dates are our anniversary, and the kids birth dates. I didn't realize it, but here's the FU... I jokingly said "what about April 16, 2003?", I literally just threw out a random date. She asked what that date was, and I just said "think about it, you'll figure it out." A couple days later she asked me again, and my daughter was in the room. I said "you haven't figured it out?!". My daughter quicky caught on that I was just messing with wife, and she agreed that it will come to her why that date is important. A couple of days went by again, and wife has the shadow box on her desk about to take it apart. She was crying because she just couldn't figure out why that date was so important that me and my daughter had known about it. We felt really bad and fessed up that it was just a random date and we were messing with her. So now every April 16th, we have a little celebration for absolutely nothing.
TLDR: Threw out a random date telling wife it was important to our family, she couldn't figure out why and cried a lot.