Some good stories! Luckily to date, I have yet to forget the plug (knocking on wood here as I type).
One simple, silly, and relatively cheap one, thankfully. Trolling out in Bamfield and was a busy time with manning rods, battling some wind, trying to stay on tack, hurry to get the lines back down, etc. Maybe had a beer or two as well? Anyways, changing out flasher and anchovy head for a favourite set up. Undo the gear on the line and toss into boat. Get distracted listening to buddy on other side of the boat and as I turn back to my side to set gear, rod is in one hand and new flasher/teaser head combo in the other. Throw the flasher overboard and slowly see it sink away. Oops - jackass forgot to attach flasher to mainline. There goes $20.