You know about the info_notnull explosion hack. You know about making monsters drop weapons or alternate ammo using the .weapon and .ammo_whatever hacks. You know about making items float in the air by spawning them on a temporary platform. Let's have a thread where we talk about new ways to use existing behavior and get novel gameplay. If you're a "retired" mapper, this is a great time to do some armchair level design and suggest ideas you'll never have a chance to use yourself.
