Anyone who owned a Sega Genesis knows the biggest POS they ever purchased was an accessory to that system.
The Sega Activator - an octagonal floor device that allows a person to use their arms and legs to control the character instead of their thumbs. Seen about halfway down the page
here.
I don't remember how old I was when I asked for this thing for christmas, but I was one heck of a sucker. you had to put the thing together every time you wanted to use it because it was too big just to leave laying around. it never did work correctly... your character basically sat in one place, randomly throwing a kick or a punch, never really coinciding with yours. you had to use it in a room with nothing on the ceiling or it went nuts - fans really screwed it up. i think the thing used to cost damn near $100 when I got it. fortunately years down the line I was able to get $70 for it out of a pawnshop owner when I sold all of my genesis stuff to buy a Playstation.
I had the Menacer too... also seen on that page. Quite possibly the worst light gun ever made. It had a pretty cool design (or so I thought when I was 12), but it didnt shoot anywhere near where you aimed, no matter how many times you calibrated it. It was cordless and the batteries died frequently.
Is it any wonder why Sega isn't in the console race any more?