false alarm

A warning sound (such as the call of sentry, the ringing of a bell, or the shriek of a siren) which turns out to have been erroneous.

(by extension) A thing or occurrence which initially causes fear, distress, etc. but which is subsequently recognized as being no cause for concern.

A person who pretends to be more accomplished or a thing that seems to be of higher quality than is later found to be the case.