1) The Pyro's initial "ambush class" design in the earliest days of TF2, prior to stuff like airbursts. The idea was an close quarters ambushing class, like a Spy without an instakill option, but the reality is that any burst damage class could effectively kill you by just... turning around and shooting you, ambush or not, which makes the whole "close quarters" thing just a weakness on the Pyro's part rather than a specialty if other classes could do more effective close quarters damage.
2) The biomods and universal ammo systems in Deus Ex: Invisible War. Biomods were meant to give players more freedom than nanoaugs in DX1, where you were locked in once you picked the aug for a body part; instead, you can swap out for a new mod as soon as you found any biomod canister. But, if you swap them out, all of your upgrades to your previously installed mods are thrown out, and on top of that, there are rarer "black market" mods that you might not find until you've already got upgraded mods installed. As a result, you end up just sticking with your initial choices anyway because otherwise it would feel like losing progress, especially if the new mod turned out to be less useful than the old one.
Universal ammo was meant to allow you to use rarer or more exotic weapons without having to scrounge for rare ammo or play inventory Tetris. In reality, it meant you almost never used those weapons because suddenly using a sniper rifle or rocket launcher a couple times means you just halved your ammo pool for your pistol or assault rifle.
3) The red zone in PUBG is a mechanic where a large red circle appears occasionally on the map in a random location, and you have a short time to get to safety before a mortar strike begins, randomly showering the area in explosions. The concept was to force players to move, look cool, and provide audio cover to people sneaking around. The reality is, a redzone blast will kill/down you in a single hit, you're completely safe inside buildings, and it's excessively loud in a game where audio is vital, meaning that 99% of players will just camp in a house (rather than moving) until it goes away rather than risking an instant death or getting into a fight with someone they can't hear moving around. The worst part is that it tends to be around or near the central areas of maps where the most fighting and buildings are, so people can just be stuck either constantly listening to nearby explosions or being stuck in a building waiting to play the game again. It's turned into a huge meme where the devs seem too stubborn to take it out or rework it beyond occasional volume/duration tweaks. They sorta relented by creating a new map with mini redzones called black zones that specifically targets and destroys individual buildings and killing people inside them, which at least actually accomplishes the original goal.