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...