mare's-nest

A mare's-nest is a discovery that turns out to be worthless or illusory—something initially exciting that proves to be a hoax, muddle, or nothing at all.

The phrase plays on the absurdity of finding a horse's nest, since horses don't build them: The supposed breakthrough was just another mare's-nest.

It can also mean a confused or disorderly situation: His desk was a mare's-nest of papers and cables.