[SDriver]

I actually enjoy it when the "thing on the cover," e.g., the vampire, doesn't make an appearance in the module. It throws the players off.

"There's going to be a vampire any minute now, he's on the cover! How the hell do we fight a vampire with a clam?"

"I dunno, check Clambook: Nosferatu."

...

Another example is Tomb of Horrors, where both versions of the cover function as red herrings.

I think it's far more of a pisser when the climactic encounter is revealed on the front cover, out in the open for everyone to see on the shelf in the gaming store. Q.v., Forge of Fury. The end boss in that module wasn't much of a surprise if you left him as published.