Help tracking down a bug

[J. Garrett Morris]

Hello,

For the past couple of weeks, since downloading 6.4.2, I've been
running into occasion panics:

ghc-6.4.2: panic! (the `impossible' happened, GHC version 6.4.2):
        tcIfaceTyVar a{tv}

Consistently, changing certain files causes this error.  Which files
in particular cause it vary by machine - unfortunately, I don't have a
very wide variety of machines to test it on.

My concern is that this is occuring in about 5000 lines of proprietary
code, and I'm not sure where to start in trying to create a repro
case.  Any guidance?

Thanks.
 /g