"unknown architecture" bug

Any idea what would cause this:

ch --make Train.hs -fglasgow-exts -osuf sun4.o -hisuf sun4.hi
ghc-5.04.2: chasing modules from: Train.hs
Compiling NLP.PTBDefs2     ( /nfs/isd/hdaume/projects/NLP/PTBDefs2.lhs,
/nfs/isd/hdaume/projects/NLP/PTBDefs2.sun4.o )

/nfs/isd/hdaume/projects/NLP/PTBDefs2.lhs:221: Warning: Pattern
match(es) are overlapped
    In the definition of `fixLex': fixLex (TDash : (TRP : xs)) = ...
Compiling NLP.PTBParser2   ( /nfs/isd/hdaume/projects/NLP/PTBParser2.hs,
/nfs/isd/hdaume/projects/NLP/PTBParser2.sun4.o )
Compiling Train            ( Train.hs, ./Train.sun4.o )
8:55am moussor:Paraphrases/ touch Train.hs
DING! moussor:Paraphrases/ ghci Train.hs
   ___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 5.04.2, for Haskell
98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package base ... linking ... done.
Loading package haskell98 ... linking ... done.
Loading package lang ... linking ... done.
Skipping  NLP.PTBDefs2     ( /nfs/isd/hdaume/projects/NLP/PTBDefs2.lhs,
/nfs/isd/hdaume/projects/NLP/PTBDefs2.sun4.o )
Skipping  NLP.PTBParser2   ( /nfs/isd/hdaume/projects/NLP/PTBParser2.hs,
/nfs/isd/hdaume/projects/NLP/PTBParser2.sun4.o )
Compiling Train            ( Train.hs, interpreted )
/nfs/isd/hdaume/projects/NLP/PTBDefs2.sun4.o: unknown architecture
ghc-5.04.2: panic! (the `impossible' happened, GHC version 5.04.2):
        loadObj: failed

Please report it as a compiler bug to glasgow-haskell-bugs@xxxxxxxxxxx,
or http://sourceforge.net/projects/ghc/.



This has happened to me a few times and by removing the offending .o file
and rebuilding, everything works (sometimes).

--
 Hal Daume III                                   | hdaume@xxxxxxx
 "Arrest this man, he talks in maths."           | www.isi.edu/~hdaume