Earlier today I started getting some ridiculous behavior out of Flex Builder. All code completion revolving around the Flash package, especially events, just quit. Then it started flagging the word "flash" itself as an error:
1120: Access of undefined property flash.
After a while hitting up the bug database and searching the googleverse I found this great tip from Garth Somerville - make a new workspace.
It's kind of a pain, but it worked!
WHEN THIS FAILS: When this fails, as Andrew T pointed out in the comments, re-install the SDK
AND WHEN THAT FAILS: Re-intsall Flex Builder. After re-building my workspace 5 times with no luck, reinstalling the SDK, and fighting with this for hours, I finally just scrapped all of that and re-installed Flex Builder. It all works fine now - old workspaces and all.