Single quotes in Haskell may be used in character literals:
a = 'a'
They may also be used in names:
f' x = x + 5
In this second case, GeSHi mistakes the single quote as the start delimiter of a string and highlights the remainder of the code as a string. The `'` character should not trigger string highlighting unless it is immediately followed by a single character and another `'`.
I've attached images from the GeSHi demo showing the second broken case.