Few more to add:
1. Fixing a flat without pulling the tire to inspect for debris that caused the puncture in the first place. Still make that mistake sometimes.
2. There's something weird about the combination of leg warmers and bibs that causes some extremely unpleasant chafing on very sensitive parts of my anatomy that is best avoided by using balm or cream, but I've forgotten that a few times before going out for 50+ mile rides, not noticing the pain because everything is numb in the cold, and then having an absolutely excruciating experience in the shower for the next few days. This is also something I still manage to do every year.
3. Forgetting a multi-tool, and then have some niggling thing that needs to be tightened or adjusted halfway through a ride like a loose cleat or slightly askew bars.
Ect.