I've had too many to recall, but here are a few:
1). I used to be a fairly fast sprinter. I was a state finalist and ran for a DI school, and so on and so forth. I always dreaded the day that I got so old and out of shape that a girl beat me a sprint race. That happened a couple years go for the first time. I was running the 100m at a low-key all-comers meet where guys and girls were often in the same heats. A high school (or maybe college) girl was in my heat and she beat me handily. I'm sure she thought nothing about beating some old guy, but in mind it was like, "Oh God. I'm slow now."
2). A few years ago I joined a running club. I had told all these guys I used to be a college sprinter, told them my times, etc. These guys were all distance runners and none had ever run on a college team, to my knowledge. One day after a run we decided to have a little 400m race on the track, for fun. I did not win. Granted, I was a little older than these guys and I hadn't done any real sprint training in a long time. But it was still fairly embarrassing. I think the winner ran 53-something, and I ran 54/55. Then we decided to do a 200. I lost again, to a different guy. Officially washed up.
3). Shortly after college, I took up distance running and decided to train for my first 5k. I had never really been a distance runner, so running 25-30 mpw or so was a lot to me. I really thought I was doing some serious training. By this point, I even had a sub-5 mile to my credit, so I figured I was ready for a 5k. A buddy of mine who ran XC in high school agreed to run with me in my first 5k race. He hadn't trained in years. I hit the first mile in 5:30 or so and put a bit of distance between me and my friend. I was feeling good and figured he was toast. At around the 2 mile mark I hit the wall BIG time and slowed to a crawl. At about 2.5 miles or so he passed me and would go on to beat me by a good 30 seconds.