These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Fridge Brilliance: The waterguns kill monsters because they are filled with holy water, and all the monsters are weak against holy symbols.
Game Breaker: The Monster Potion, which grants a few minutes of Nigh Invulnerable, and also has punches that can destroy ALL but a few bosses very quickly and that can break through walls and doors.
Password "BCDF" is the most useful password you can use; it takes you to an obscure bonus level based off of Day Of The Tentacle, where you can find a lot of goodies and an extra life. If that wasn't bad enough, if you have bazooka ammo left over, you can go there again! And losing no victims in either runthrough pretty much means that you can max your lives before the fifth level.