Any idea why I get a "field integrity exception" error when I try to update SOME PricebookEntry objects?  It doesn't happen for all Pricebooks/PricebookEntry objects, just some, but I can't figure out whether there's a pattern.  I'm definitely using a valid PricebookEntryId.  Any ideas are appreciated.

Any idea why I get a "field integrity exception" error when I try to update SOME PricebookEntry objects?  It doesn't happen for all Pricebooks/PricebookEntry objects, just some, but I can't figure out whether there's a pattern.  I'm definitely using a valid PricebookEntryId.  Any ideas are appreciated.