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
I was a professional game tester for 13 years, so I found some big ones in my time. But my favorite was DOOM 3 for Xbox. Triggering a cutscene caused the game to memory dump everything else, which despawned every enemy active on the map at the time. So if you memorized where the cutscene triggers were, you could sprint around the whole map, spawn in every monster, and then trigger the cutscene to kill everything without firing a single shot.