Stupid things I've done
Pay attention to your spell checker.
Be sure to review the suggested replacements by your spell checker. Spell checkers don't always suggest the correct replacement. I recently wrote to the folks at Tucows.com and didn't pay attention when spell checking. I'm sure the Tucows folks were confused when I kept referring to their service as "tacos".
Don't use your business cards as a notepad
At a Anti Spyware conference earlier this year I enjoyed a promising chat with Eric from Google. We traded business cards to keep in touch. I had written on the reverse side of my card but Eric was very polite when he returned it later that day. He didn't mention if he had read the list of things I wanted to ask my family doctor. We haven't kept in touch but I hope my name is left out when he re-tells this very funny story to others. .