another one: Writing a long, well-structured, relevant post and then deciding not to post it.
Good I don't do most of them
The few times I came close to doing that, a flag goes off in my mind that if I'm writing something "long, well structured, and relevant to something", chances are I'll want to post it eventually, so I have saved some of those as "unsent drafts" that I later fold, spindle, and mutilate into other posts.
P.S. I'll give Paul Keith a cupcake for being in my top 10 list of people who consistently write long, well structured, relevant posts!