Here are my favorite mistakes so far:
1.
My first time skippering my own boat I was thinking I should reef.
I thought the best way to decide if I should reef or not, would be, to use my
binoculars to look at other boats sailing near-by and see if their sails are reefed... That way I could avoid looking like a wimp and rely on the other more experienced sailors in the bay for their professional judgement of the wind conditions...
Their sails were not reefed so I decided not to reef. lol, 5 minutes later the keel was almost out of the water as we were drifting into a rocky lee shore.
2. single handling, motoring into the marina realizing I forgot to tie
fenders to my boat... So I let go of the tiller to get the
fenders from the cabin, boat steers straight for the shore while I'm in the cabin, I get out,
fenders in one hand, and jump for the tiller to swerve the boat to the other direction... the some part of me thought it was absolutely critical that I hurry to get the
fenders on the starboard side of the boat...
With both hands I wanted to quickly tie one
fender to the life
line with a clove hitch... instead of tying the
fender to the aft portion of the SAME life
line, I decided to WALK up to the center of the boat and tie it there, the boat veered off again to starboard and I jumped back to the tiller again, dropping the
fender overboard... DOIH.
I circled twice trying to retrieve my
fender by myself in the middle of the channel traffic and a whole water side restaurant watching me. Embaressed I left the scene quickly without retrieving my
fender 
"Next time I'll just tie the
fenders in the cockpit and slide them forward" I thought,
dumb lesson learned lol
there's many more but those are my favourite ones.