Here's stuff I've forgotten to bring in past years. Maybe it will be of value to others, especially new folks. Veteran festevarians, please add to the list! I finally made myself a checklist a couple years ago.
hammer (for tent stakes. It's the ROCKY mountains)
sharp knife (for slicing tomatoes for my sandwiches)
drinking cups
salt and pepper
corkscrew
shampoo
soap container
...and last but not least... my ticket!
Newbies: it's almost guaranteed to rain at least for an hour one afternoon. Sometimes more. Bring an umbrella and a rain jacket or poncho. And it can get cool at nights, so bring a fleece jacket or equivilent, jeans, socks, cap. And a flashlight.
For some reason, I've never forgotten to bring beer.
Listening to:
Rage Against the Machine