Aha! The first reply that actually provides an answer to my question. Many thanks. Could you say any more about this, or perhaps provide an example? Its hard to know what you mean.
EDIT: Attempting to answer my own question, perhaps it occurs when you define a product type with a constructor
data Foo = Foo bar baz
and then you want to add a new field to Foo
data Foo = Foo bar baz quux
All your explicit uses of Foo in construction and pattern matching have now broken.
"The spec changed, and I now have to be able to handle geese, where I originally typed for ducks."
There are ways and tools to minimize the impact, but it's still a lot extra overhead to think about and deal with.