How horrible! What an anticlimax!
The Visual Thesaurus's Word of the Day is anticlimax, which they define thus:
Not much head-scratching accompanies the attempt to unpack this word, which denotes an event or outcome considerably worse than what was expected.What? Oh, I see what they mean.
But their definition would fit this, which is what I first thought of:
Geologists had predicted a minor event, but instead the volcano exploded into a pyroclastic cloud that killed thousands. What an anticlimax!I think I'd have chosen a word other than "worse" had I been they...