I created this trap as a way to make a dungeon more variable- if the players were able to avoid it, their trip to the final boss would be somewhat less dangerous, or dangerous in different ways. It also represented the villain's first line of defense against intruders.
The flusher depends on three Decanters of Endless Water, and a large hinged trapdoor. On arrival at the dungeon's front door, the party was presented with a combination lock - tap the colored circles in the right order and the door opens. Fail to do so in three tries, and three Decanters set into the floor begin to inundate the room.
With successful swim checks, a hero might be able to get out, however the trap door will come into effect in another three rounds.
Assuming the players fail to guess the combination, the water fills the room and once a certain weight threshold is reached, the trap door opens downward, discharging the heroes (and a lot of water) into a water filled pit on the edge of a swampy dungeon level.
In my case I placed a large carnivorous plant to waiting to grab anyone flushed in, and a young adult black dragon which was brought here and left as a guardian by the villain. These challenges can be adjusted based on the party level, and the initial "flushing" effect need not be lethal as the pit could be changed to a flat dungeon floor, replacing the risk of drowning with a small amount of falling/crushing damage.
During my session the party managed to sidestep the trap once(one player used insane Batlogic to correctly guess the combination) but there is no reason this trap could not be re-used if it fails to be triggered.
So please, steal my trap for your home game, and let me know how it works for you.