My two experiences with forgetting the plug are very different from one another. The first was a hot summer day a few weeks after getting my first boat of any significance of my own. A 16' Starcraft runabout with a 50hp Johnson. My wife, her mother and her aunt asked to go for a ride now that I had been out a few times and had gotten most of the bugs worked out. So, we left the beach house and went to the boat ramp a few blocks away. Got everything loaded up, launched the boat and off we go. Boat seemed a little sluggish and unresponsive. Figured it was just from having more people aboard than I was used to, until our feet started getting wet! That was the last time my mother in-law went on a boat ride with me...
The second time was when a guy cut me off at the boat ramp in the morning headed out duck hunting. I was backing down the long ramp and he wheeled in, cut me off and launched first. Then proceeded to unload his truck and load the boat while blocking the ramp. As politely as possible I asked him to move his boat out of the ramp area so I could launch. After I launched and was leaving the ramp we noticed he was unloading his gear. As we motored past by buddy commented that he had forgotten the plug and all the gear in his boat was floating! The guy ended up loading back up and leaving, never went hunting. Karma can be a bitch!
I've come close to forgetting a few other times but my prelaunch mental checklist usually kicks in ad saves me from myself.