I recently posted about Perian, “the Swiss-Army knife for Quicktime.” Apple has posted a document that may be of interest if you have been having trouble with AVCHD format if you have Perian installed. From the Apple support site:

Issue

Final Cut Pro 6 may unexpectedly quit when attempting to transfer AVCHD media, if the third-party Perian QuickTime component is installed.

Products affected

Final Cut Pro 6.x
Perian
Solution

Check to see if the Perian QuickTime component is installed:
/Library/QuickTime/Perian.component

Drag the component to the Desktop or to the Trash, and test for the issue.

As always you should always install and 3rd party add-on with caution and never install or upgrade in the middle of a project.