How about this common mistake:
Going forward to check something. Returning to the cockpit and realizing my mobile phone is no longer there (I have done this one twice).
I did this one on my old Catalina 22 with outboard:
Row to boat, which is on mooring. Prepare to get underway by removing sail covers, etc. Start engine, drop mooring
line, motor about 50' when the engine dies and won't restart. QUICKLY raise sail so you don't hit anything in the crowded harbor . SLOWLY sail while cursing and trying to start outboard. Then remember to open the vent on the gas tank.
And of course this one:
Raise the
spinnaker on a mild breeze. Enjoy a wonderful sail in calm conditions. Then turn to head home and realize that the calm conditions are really a 15 kt breeze when headed upwind! Not terrible, but the rookie crew was less than happy with the boat heeling 20 degrees and pounding in the chop that developed.
Barry