1. Marlin XT-22
I got this years ago when I wanted a tube fed bolt action .22 rifle. I watched some videos and read forms where people were all talking up the microgroove rifling Marlin uses in their .22's and how accurate theirs were. Me, not wanting an inaccurate rifle bought it. $270 total price, tax included and nearly a month long wait to get it. That was the last time I ever bought a gun from an LGS, ever since I buy online and the guns show up within a week, guaranteed.
The rifle shoots like shit. I can't get under 2 inches at 50 yards with any ammo, but I take my Ruger Charger to the range and it's half an inch at 50 all day with any ammo and much smaller and lighter than the XT-22. The stock on the XT is the cheapest f***ing plastic I've ever felt, the iron sights are terrible, it's unwieldy unless fired off a rest. It's got a nice Accutrigger, but that's about it.
Some say that Marlin has improved their quality, but I'm not taking the chance. The brand is dead to me, as is Remington.
2. Leinad over/under .410 derringer.
My two biggest problems with this gun were the sights and the safety concern. The safety issue is, because it's an over/under and not a side by side, the hammer contacts BOTH firing pins when the selector is flipped to fire the top barrel, when it's selector is set to fire the bottom barrel, it only touches the bottom barrel firing pin.
So, it's safe to shoot if you fire the bottom barrel first. Now, I've tried to fire the top barrel first with the bottom one loaded and I've never had a successful double discharge, but different primers and ammo could make it possible and I don't want to find out what happens if it does.
The front sight in the pictures before I bought it looked like it was a normal sight, but when I picked it up at the store, I saw the front sight is like a triangle, so when lining it up with the rear, you have no idea where to hold it. The single shot and side by side has a normal front sight that's square.
Besides those things that only affect the o/u model, the trigger is shit, but I knew that when I bought it and that's across all Leinad/Cobray derringers. The O/U is really flat, so it would carry well (one reason I bought it over the side by side) tho it is heavy.
And while this is a top 2, another gun I wish I hadn't bought was the first one I ever bought: A Mossberg 18" over/under. For $550, better shotguns can be bought, but why this isn't in my top two is because even though it's overpriced, it's still fun to have a double barrel shotgun.