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

http://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. Using the utility program Recover.exe is the recommended course of action. Exporting and re-importing the file will work however may result in more lost data. The Export program will only read to the "bad spot" on the file and stop whereas Recover.exe will continue to the end of file reading and writing records beyond the error. Information on this utility can be found at the above address.