The following file causes GHC to crash, if compiled in the "C" locale.

$ LC_ALL=C ghc unicode.hs
[1 of 1] Compiling Foo              ( unicode.hs, unicode.o )

unicode.hs:2:1:
    Warning: Pattern match(es) are overlapped
             In an equation for `<stderr>: hPutChar: invalid argument (invalid character)

unicode.hs:

module Foo where
δ x = 3
δ x = 4