(From http://www.haskell.org/pipermail/haskell-cafe/2009-December/070193.html)

In the attached code,

1. if you load the code into ghci and evaluate e it will hang, but (defaultValueD dict) :: Expression returns fine
2. if you change the gunfold instance for Proposition to, error "gunfold" it stops hanging -- even though this code is never called.
3. if you change, ( Data ctx [Expression], Sat (ctx Expression) => Data ctx Expression, to (Data ctx Expression, ....) => ... it stops hanging.

If someone could explain why each of these cases perform as they do, that would be awesome! Right now it is a big mystery to me.. e calls dict .. and there is only one instance of dict available, which should call error right away. I can't see how something could get in the way there...