Not getting it in writing. It doesn't have to be a long drawn out contract, just a short email explaining what the agreement is. Even amongst friends, you will never regret having an email stating your agreement.
Trusting the level of integrity, gratification posed by founders/managers and slogging my ass for their success. Reminds me of Austin Wiltshire's blog on New hire Cannon fodder.
Trusting the wrong people is painful - even when you have it in writing.