Word (or phrase) that means something you thought was a great idea, that turns out to be a massive error

We have a variety of suggestions for something that you thought was bad, but turned out to be good.

Is there a single word that describes something that you thought was going to be a brilliant idea that turns out to be an apocalyptic blunder.

"His idea was an x"

Solution 1:

See goldbrick

odo

A thing that looks valuable, but is in fact worthless.

Solution 2:

The first comment on the question you link to suggests “a blessing in disguise,” which made me think of a wolf in sheep’s clothing, which refers to someone (or something) that initially appears benign, but is ultimately revealed to be harmful.

I’m not sure how well this fits your use case, but bait and switch seems to be related:

a selling method in which a customer is attracted by the advertisement of a low-priced product but then is encouraged to buy a more expensive one

i.e., something appears to be a bargain (a good deal), but you end up getting a worse deal than you expected.