Bugs:Maniac Mansion (NES, USA)
This page details bugs of Maniac Mansion (NES, USA).
Freeze Nurse Edna
The game's AI is not the best: it uses player-character entities to control many of the game's NPCs, including Mark Eteer, Sandy, the Tentacles, and the Edisons themselves. The game's control over moving these player pawns in real time noticibly taxes the NES' CPU, is used sparingly (the scripts just force the family members to teleport most of the time), and most importantly, can easily be broken by switching characters.
Perhaps the easiest way to freeze Edna is a very simple one - first, have two party members inside a room on the top floor. Next, move the "expendable" party member into Edna's room. Let Edna proc her script to approach you, then immediately switch kids to the other party member. Wait 1-2 seconds, then exit into the hall, and Edna should give chase to your current character - simply go back inside the room you were in. Edna should be "confused" and frozen when she returns to her room, the AI logic for arresting the expendable character now null and void. This can also be used when returning from Edna's loft room: simply perform the trick again.
This trick can also be used on Weird Ed with a slightly different variant. You still do the initial switch, but he'll usually not leave the room after the "capture", and you can go inside and immediately leave to call the attention off the expendable party member. This trick is useless with Weird Ed, however, since he'll still react to anything you do in the room.
Universal Inner Door Code
The inner door code for Fred's secret lab is supposed to be random (or pseudo-random, at least), and the player is expected to go through a series of tasks that allow them to play the Meteor Madness game after Fred does to see his high score which, coincidentally, is the inner door's code. However, the game doesn't set the value for the code until all the prerequisite tasks are done and Fred plays the game, leaving it as 0000. As such, if the player never fixes the broken wiring, Fred can never play it (he'll try to play but can't, and his attempts don't change the code in memory), and the door code will always be 0000.