You can also get this error if a file is damaged. If a {file}.dat is damaged and you rebuild the {file}.key from it, you may or may not get an error message indicating that there's a problem in the .dat. If you get no message, you could be left with the impression that your .dat/.key pair are good. Running verify may or may not tell you if there's a problem. (This is a low probability, but possible scenario.)

Your safest course of action is to run through the "What to do with a file That Fails Verification" procedure in

www.appx.com/assets/asp/dynamic generator.asp?pageid=624.

You can do a bit more investigation using the techniques in "Verifying and Analyzing a File" on that webpage