Flash CS4 refuses to let go

Flash still has the ASO file, which is the compiled byte code for your classes. On Windows, you can see the ASO files here:

C:\Documents and Settings\username\Local Settings\Application Data\Adobe\Flash CS4\en\Configuration\Classes\aso

On a Mac, the directory structure is similar in /Users/username/Library/Application Support/

You can remove those files by hand, or in Flash you can select Control->Delete ASO files to remove them.

Try deleting your ASO files.

ASO files are cached compiled versions of your class files. Although the IDE is a lot better at letting go of old caches when changes are made, sometimes you have to manually delete them. To delete ASO files: Control>Delete ASO Files.

This is also the cause of the "I-am-not-seeing-my-changes-so-let-me-add-a-trace-now-everything-works" bug that was introduced in CS3.

What if you compile it using another machine? A fresh installed one would be lovely. I hope your machine is not jealous.

I have found one related behaviour that may help (sounds like your specific problem runs deeper though):

Flash checks whether a source file needs recompiling by looking at timestamps. If its compiled version is older than the source file, it will recompile. But it doesn't check whether the compiled version was generated from the same source file or not.

Specifically, if you have your actionscript files under version control, and you Revert a change, the reverted file will usually have an older timestamp, and Flash will ignore it.