scavenge_mutable_list: strange object?

[Johannes Waldmann]

I sometimes get this error:

>  internal error: scavenge_mutable_list: strange object? 3

( large program, running on debian linux on i368, compiled with 
ghc-6.0 -pgmc gcc-2.95 --make  -O2 -fvia-C -funbox-strict-fields )

what can I do to a) work around this, b) help finding/fixing the reason?
-- 
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