Maniac Mansion Expert

[Kennedym]

What kind of actions will prevent you from being able to finish the game?

[Radiant]

Dying, mainly.
(in the original, you didn't get a character's stuff back when he died)
Also, wasting dimes so you can't use the telescope.
Calling the MP too often so he won't come again.
Messing up the deal with Mark (either Green's or the Meteor's).
Messing up Ed's film.
Tearing open the envelope, or forgetting the stamp.
(the above four only apply if you have no other way into the Seckrit Lab)