Ticket #7512 (closed bug: worksforme)
GHC crashes with multiple => in instance
GHC will crash (rather than a syntax error) if you write
import Data.Array.Unboxed newtype Vector r = Vector (UArray Int r) instance (IArray UArray r) => (Num r) => Num (Vector r) where (Vector v1) + (Vector v2) = Vector $ listArray (0,0) 
This is unfortunately the smallest program I found that crashes the compiler; removing the definition of (+) yields a syntax error as expected.