Here's what I was confronted recently due to a broken sandbox environment:

$ cabal info parsec
cabal: ghc-pkg: Data.Binary.Get.runGet at position 9583: demandInput: not enough bytes

The main problem is that this message is not very helpful (other than telling that ghc-pkg wasn't able to do something). Duncan told me, that the recent binary API now provides better facilities for error reporting. That could be used to improve the UI of ghc-pkg when reporting errors.