Things that I forget (or things that happen) because I often get up before 5am during the week to run. I'll go out for anywhere from 4-11 miles then head to work and shower, starting work around 7am:
I've forgotten work clothes (socks, shirt, underwear, somethingt to comb my hair). I've had to drive back home from work (after showering) in my sweaty running clothes. I've bummed a work shirt from our Marketing Department. I can get by without a comb or brush, it just makes my hair a little poofy. After a few times of being forgetful, I started storing an extra set of clothes in my locker there.
Last week - something new happened. My 11 mile run on a Thursday, filled up my two water bottles, placed them in my back containing work clothes. Got to Oak Grove Lake - and one bottle was Completely empty. Ugh. Top was on and the spout was closed, but the top was not tight. Nowhere close to being on tight. So - one bottle of water I made do with, and the extra set of clothes came in handy with one exception. I do not keep a 2nd pair of Work shoes in my locker. It was 27° that morning and I was not going to wear wet shoes (no extra socks either, but those weren't so wet). I ended up wearing my running shoes - luckily, I had worn my gray trail running shoes, which didn't stand out as much with my black pants as my mostly white ones would have.
So - be careful when packing your things the night before a run, or Early in the morning. And it never hurts to have a extra set of everything for when you do mess it up. If only getting more sleep was an option...