When I was playing dying light with friend we discovered that every timw you go to sleep and wake up air drop will appear, if you don’t know air drops are pretty rare and if you collext them and deliver to one guy you will recieve lots of xp, we used that multiple times to unlock grapling hook very early. Air drops also gave lots of medicine which was hard to get in early game and that very helped us
Pokémon Red, exploiting the multiplying item Glitch along Missingno.
Killing Floor, I remember some Maps back then where you were able to stand safe the entire rounds. Yet, I did not use it myself only once but felt unfair and well, exploitable too much in an online game, obviously.
Other than that, random games with bosses that had dead blind spots to stand by, mostly older games. For example, Early 3D games. But can’t put my finger now on any specific one.