I once left my seat bag at home...with all my flat-fixing gear. Went on ride anyway. Thankfully, no flats! The one time I did get a flat, I discovered that I only had 1 CO2 cartridge, rather than the two that I thought I had.

I also left my water bottles in the car once. I wondered why the bike handled so nicely. Luckily I realized it less than 2 miles in, so it didn't take me very long to get back to the car.