Just thought I'd write this here as I just came across it in Colin Moocks book, and haven't seen it mentioned before - could be wrong...

Anyway - really a damn handy undocumented feature with flash 4 and 5:

With user input fields, you can detect when the content has changed (much like in HTML)

name a text field - eg. someText

in the same timeline, label a frame with the text field name followed by _changed

eg. someText_changed

That frame will be called ( a la the old flash 4 Call action) anytime the contents of the text field/input text is changed (with keystrokes) - so you can put any code you like in the frame to operate when the user types new stuff in. Like set an 'is dirty' flag, or whatever - so you can auto-save on change, or warn when data hasn't been saved after it's changed etc etc.

Can't believe it's an undocumented feature, but there you go.