Quote Originally Posted by jp498 View Post
The easiest mistake for me to make is to not set the aperture in #4. Essentially shooting wide open instead of at the proper aperture.
Me too. I'm also very good at taking long exposures on T instead of B, so that the shutter is unbeknownst to me still open after I release the plunger.

Failing to reverse the darkslide before reinserting is another good one.