Always check ISO, and all setting for that matter, before starting to shoot.
I set my fn button to bracketing (typically used for ISO) and forgot to check what it was set at. I have no clue why, but my ISO was set to 5000...
I realized this with 5-minutes left in a sunset photo shoot tonight. Needless to say the grain is through the roof and the shots are unusable. Thankfully the sun sets every night and I'll get another crack at it tomorrow. I'm usually Jonny on the spot about this but was helping a new photographer get up and running. No excuse though.
NOTE TO SELF: Check setting before shooting elevnty-billion photographs. Regardless of what's going on around you.
Check out my flickr photostream: http://flic.kr/ps/UGiTJ