Wow I have a couple of these, most of them harmless and couple that I really regret.
In the harmless but "Doh!" category I think leaving the boathook unsecured on my last Charter in the BVI was a doozy. With just 2 of us on a Bene 39 it made for some Mooring drama. I found myself on the dingy politely asking others to borrow a hook while the GF was doing figure 8s. It was blowing a bit so I decided against the dingy securing assist method and we ended up ok, but wow was that a lot of trouble caused by such a little mistake. I rigged a new boat hook by taking the paddle off an oar and using a couple a coat hanger hooks with duct tape. Worked great!
On the more serious side would be pulling behind a large cat as directed by the dock staff onto about 5 feet of dock. That wasnt so bad, but I actually jumped away from the helm on to the dock to secure a line, leaving no one at the helm. It didn't lead to a huge fiasco, but it really hit me as one of those, "Wow, that could have been really bad" moments and for some reason is the only time I seemed to panic. I realized my error quickly enough when I saw that no one was aboard, the engine was idling and 30+ feet of someone else's boat started to swing out away from the dock. Really not that big of a deal, but it could have been. Others may have a different view, but mine is do not ever leave the helm if that is your job.
Lastly as a kid and filling in as middle man on my dad's etchell for a season I learned the hard way how not to pull in a sheet. I broke 2 fingers in a self tailoring harken. Fist over fist is natural now, but man was that a rough school year. Hah.