Only 2 things really come to mind:
1. During my European funfair/theme park tour in Sep-Oct. 2015, I took the train too late to get to this fair in western France, and by the time I got there the fair had just closed (like 10 minutes earlier). There was a rare flat ride there called Stargate I wanted to ride. The operators were still there, and I remember asking through the fence if I could ride (hey, it was worth a shot). Nope. Here are videos of it:
Offride:
https://www.youtube.com/watch?v=S0Y_t7nALAM
Onride:
https://www.youtube.com/watch?v=B60Od57FPXw
2. Not visiting Cedar Point, Knoebels, Magic Mountain, SF Great Adventure, etc. before Covid hit. Of course in 2019 I had no idea Covid was coming, so it was easy to put these parks off "until 2020 or 2021." Um....I mean, you could still visit these parks in 2020 or 2021 (well, not Magic Mountain in 2020), but it won't be nearly the same as 2019 or earlier.
I already kind of appreciated this pre-Covid, but Covid has made me appreciate more that you need to tick every box on your "bucket list" ASAP. You never know what will happen in the world (pandemics - natural or manmade, world war, world-changing natural disaster, governments restricting freedoms, etc.) or how much time you have left on this earth.