I got bored, thought I would play some Underhell since it's one of my favourites and I had not played it in a while.
In uh_chapter1_02 map after signing the first checkpoint in the game, if you go too fast to the canteen, the guard you are supposto follow will get stuck in the room with the checkpoint. (you can walk back and if you hug the door he can walk through it to the otherside)
You can trap NPCs (works best on guards who only have batons) with different props like crates and laundry carts. Was pretty useful when trying to save Todd, just put a gas canister in front of the vent and you don't even have to kill the zombies.
You can fly with the flare box in the house. (place it next to a wall, get on top of it and pick it up.)
There are also some places you can get to just by stacking crates and other things on top of eachother, like the catwalk in canteen. Haven't found any particularly useful ones yet.
The cypher can go inside you and make you get stuck forever.
Survivors getting stuck trying to pick up weapons out of reach.
Also survivor Joseph is pretty much 100% not going to stay in safe room and will keep following Jake.
Holding down reload and shoot sametime with sniper rifle enables rapid fire destroy everything in your way mode.
Spamming ironsight let's you rapid fire shotguns.
Final one for now, Quilt in dreams after chapter 1 simply being stuck forever in front the cabinet when hiding.