If

import Test.QuickCheck

prop_eq_reflexive x = x == x

is loaded into GHCi the following can be observed:

*BadErrorMessage> quickCheck prop_eq_reflexive

Top level:
    No instance for (Show (IO ()))
      arising from use of `print' at Top level
    Probable fix: add an instance declaration for (Show (IO ()))
    In a 'do' expression: print it

Hugs gives the more understandable error message

BadErrorMessage> quickCheck prop_eq_reflexive
ERROR - Unresolved overloading
*** Type       : (Eq a, Show a, Arbitrary a) => IO ()
*** Expression : quickCheck prop_eq_reflexive

:t in GHCi gives an even more understandable error message:

*BadErrorMessage> :t quickCheck prop_eq_reflexive

<interactive>:1:0:
    Ambiguous type variable `a' in the constraints:
      `Arbitrary a' arising from use of `quickCheck' at <interactive>:1:0-9
      `Eq a' arising from use of `prop_eq_reflexive' at <interactive>:1:11-27
      `Show a' arising from use of `quickCheck' at <interactive>:1:0-9
    Probable fix: add a type signature that fixes these type variable(s)

(In the introductory FP courses at Chalmers we use Quick``Check from the start, so this error message bites students in the first week.)