The former is often a product of the masking effect of the avatars - the temptation is just too great. The latter is more pervasive since the MUD illusion relies on the user's willingness to be taken in. The user buys into MUDding despite its obvious contrivance. Disbelief is the price of admission and users leave their doubts at the door. Analogies drawn by Brenda Laurel between computers and theater are extremely valid here.
Skepticism is a defense mechanism, the nagging voice of reason.
It's what helps us separate fact from fantasy. It's exactly what we don't want at the theater - or a MUD for that matter. But by abandoning doubt MUD players risk betrayal. These lapses may be accidents or pranks of programmers. But the rewards of simulated society obviously outweigh the hazards for most players.