1. Summarise the issue (what were doing, what went wrong?)

I was recording some changes, before noticing at last regrets that I
don't want to include a particular sub-hunk that I've already said yes to.

I have already recorded a file containing:

    -- a comment

    data D = C Int
           deriving (Eq, Show)

which I then change in working to:

    -- another comment

    -- a comment

    data D = C !Int
           deriving (Eq)

Then, in record, I don't record the initial comment-addition, I do
record the final hunk.
I get to last regrets before realising I don't want to remove D's Show
instance,
so I put it back using the hunk editor. I accept the remaining changes,
but darcs fails:

    WARNING: Doing a one-time conversion of pristine format.
    This may take a while. The new format is backwards-compatible.
    Pristine conversion done...

    darcs failed:  ### Error applying:
    hunk ./file 5
    -data D = C Int
    -       deriving (Eq, Show)
    +data D = C !Int
    +       deriving (Eq)
    ### to file ./file:
    -- a comment

    data D = C Int
           deriving (Eq, Show)

darcs shouldn't fail, and should record a patch that simply adds a ! to
the Int parameter of C.

And the obvious thing to point out is that the pristine conversion
message is spurious, and any fix of this bug should look into why it's
occurring!

To my great surprise I can still reproduce this bug.

Indeed we have a failing test case for it.