Love and Kisses, Zaphod

In the film of "The Hitchhiker's Guide to the Galaxy," Zaphod Beeblebrox's stock with Tricia McMillan, a.k.a. "Trillian," tanks rather severely when she learns that he signed off on the order for the destruction of the planet Earth, thinking he was signing an autograph.

Example of:
Tricked into Signing