What annoying mistake(s) do you continue to make?

[Brian Henderson]

Oh yes...reading the ruler backwards and cutting at, say, 12 1/2 inches instead of 11 1/2...and then making 20 of something !
Scott

Of course, doing it the way you say, cutting at 12.5 instead of 11.5, isn't a big deal because you can cut that extra inch off. It's a little harder putting that inch back on.

[Mike Nolan]

My two biggest mistakes are:
Carefully making a plan and then not following it.
Trusting unpowered tools not to bite me. The two biggest offenders are chisels and drill bits.

[Von Bickley]

My list would be too long to post here.

[Julie Moriarty]

Believing my memory is correct when doubt is telling me to go back and double check.