Has there ever been a bomb threat where there was an actual working bomb found?

I was in Lake Tahoe when this was gong on.

http://www.fbi.gov/news/stories/2009/august/a-byte...

A group of men put a large bomb in a Casino Hotel and called in a bomb threat trying to extort money from the Casino. The bomb eventually went off, so it certainly was an actual working bomb.

Yes

This tactic was routinely practiced by the IRA as it allowed them to get all the terror of a planting a bomb without the bad press of actually killing women and children.

Domestically, such threats frequently occur when a friend or confederate of the bomber suddenly realizes that their friend is crazier than they thought he was and that if they don't report it, they're part of the conspiracy.

The Stern Gang, a Zionist terrorist organization, claimed it gave a warning before it blew up the Kind David Hotel. It is not clear the warning was received or directed so as to effectively warn occupants of the building. In any event many were killed in what is still the single most deadly terrorist attack in the Middle East. Many members of the gang were later in or the head of the Israeli government. Terrorism works.

Back in the olden days of the IRA, terrorists just wanted to scare people, not kill them, then the rag heads went and messed that all up.

Yes, there have been. If you watch true crime films you will see many cases.