I've had two.
My first one was when I was a teenager when I was golfing with my friend, his dad and little brother.
Trying to be funny, I took a junior-sized 5 iron from my friend's little brother's bag and used it on a par-3 200 yard hole.
It felt like swinging a putter and I hit it harder and lower than I planned to. The ball went straight at the pin, slammed into the pin, went up in the air and dropped straight down into the hole.
For the rest of the time we were out there, my friend and I had his little brother convinced it was a golf rule that if you use someone else's club and get a hole-in-one, you get to keep the club.
The second one I was older playing in a charity golf tournament my company was sponsoring. They had a 5-minute rule on lost balls and then it was a 2-stroke penalty and drop from where you last hit it.
I thought the ball went past the green which was a big drop-off along with a water hazard behind it. My group and I looked for my ball and just when I said I was just going to take the penalty and grab a replacement ball one of the guys who was getting ready to put asked me if that was mine in the hole, and it was.
The sad thing is I didn't even get to appreciate it because we had to move on immediately as the delay backed up the groups behind us.