David Feuer managed to tickle a corner case in the
code generator. See Note [Scrutinising VoidRep]
in StgCmmExpr.

I rejigged the comments in that area of the code generator
  Note [Dodgy unsafeCoerce 1]
  Note [Dodgy unsafeCoerce 2]
but I can't say I fully understand them, alas.

(cherry picked from commit 9c78d09e)