Fake door
Trapdoors (also known as Fake Doors) are special traps designed to resemble real doors. They were placed throughout Luigi's Mansion to confuse and injure Luigi during his quest to rescue his brother Mario. If the green-clad plumber happened to turn the knob of a Trapdoor, the door would spring open and flatten Luigi against the wall, causing Luigi to lose ten HP (and a few Gold Coins). A ghost could be heard giggling after Luigi was smashed. Sometimes, a trail of conveniently placed Gold Coins would lead straight to a Fake Door to catch Luigi off-guard.
To determine if a door was real or fake, Luigi could check his Game Boy Horror. Real doors appeared on the map, while trapdoors didn't. Additionally, Luigi could vacuum a door with his Poltergust 3000. If the door rattled, it was real; if the door did not shake, it was a trap. Another way to tell if a door was real or fake was to look for a doormat under it. If there was a mat, the door was real. In some cases, however, a real door like the fortune teller's room would not have a mat either. Another way to tell if the door was a fake was when Luigi walked past a fake door, he would glance at it as he walked by. Luigi did not look at doors if they were real.
The only way to defeat a Trapdoor was to set it on fire. The door will go up in flames and disappear. Real doors would not catch on fire. However, if Luigi entered two different rooms and returned, the Trapdoor would reappear.
It should be mentioned that in the blackout, the ghosts who are having a field day in the mansion increase the amount of trapdoors (like trapdoors leading to the storage room, projection room, and kitchen if they were real).
Trivia in The Legend of Zelda: Ocarina of Time there are doors like Trapdoors but they squish you differently.
