If you have a licensed Adobe product such as Adobe Acrobat Pro 9.0 and you go to install a trial version of Adobe Flex, be prepared to be hit with an error "Invalid Serial Number". 

I think the problem is that the Adobe Flex trial overwrites your serial number in your registry along with some other things that are needed to ensure your licensed product is valid.

Changing the key in your registry will not work.  You'll have to uninstall everything and re-install your licensed product.

You'd think a company like Adobe wouldn't have made such a mistake between licensed and trial versions...