Recently, I forgot to bring my rain pants for a week long tour. For three days it rained a cold rain on me every mile I was on the road until I abandoned the tour and had an old friend drive 4.5 hrs. to rescue me.
I once forgot my cycling shoes. I was on the way to an club event and realized it maybe 20 min. into the drive. Returned home and decided to do a local club ride at noon. Got five miles into that ride and realized I had left my water bottles in the car from aborted drive to the club event. It was clear I was not supposed to ride that day.
One night I camped out at the start of a club event only to discover the next morning that I had forgotten my bib shorts.