One thing I really do regret in life is never reciting pi to 50,000 decimal places. Particularly because I still come across people who credit me with the feat of memorising it, without adding the quite important qualifier that I never actually recited it, ever, and thus everybody only has my word for it that I memorised all those numbers in the first place.
For the benefit of people who haven't heard the story, this was back in 2005, I'd just won the World Memory Championship for the first time and I found myself completely and totally incapable of sitting down and training to win it again. Complete and utter mental block - my pile of packs of cards gradually fell down the back of my desk one by one whenever someone bumped into it, and stayed down there for months. There's probably still a three of diamonds down the back of the radiator in my old flat in Derby. So I decided that until I got over it, I'd do something else memory-related and break the world record for pi. The well-known story is that after several months of memorising, I'd arranged a date and place to recite it and then someone else recited 83,000 digits before I got a chance.
I still wonder whether I would have been able to do it. I think probably not, to be honest. Reciting out loud isn't a strong point for me, and however familiar I was with the sequence of numbers, I'm sure after a couple of hours in a public place, conscious that several increasingly bored spectators were following my every digit, my mind would have started wandering and I'd have got lost. But it still bugs me a little that I never made the attempt - at the time, it seemed like a better idea to give up on pi and throw myself into practicing for the world championship after all, since it was only six weeks away at that point (I came 4th, and was probably lucky to do that well), but now I wish I'd just accepted that it would have been an "amazing second-best-ever memory feat!" and done it anyway.
Because I've long since forgotten it, over the last six years. I recite 50 digits really quickly for a party piece (and sometimes 1000 digits at particularly boring parties), and I remember that the 50,000th digit was a 1, but the rest is lost in the darkest recesses of my brain's filing system.
One day, I'll finally have created by 4-digit number system, and then I'll memorise pi to 50,000 places again. And this time recite it properly and officially. Just to show I can.