For the following program (BadSeek.hs), type a couple characters of input and press return. With ghc-6.10.3, it works fine; with ghc-6.11 it throws an error.

module Main where

import System.IO

main = do   
    getChar >>= print
    hSetBinaryMode stdin True
    getChar >>= print

With ghc-6.10:

$ ./BadSeek 
ab
'a'
'b'

With ghc-6.11:

$ ./BadSeek
ab
'a'
BadSeek: <stdin>: hSetBinaryMode: illegal operation (cannot flush the read buffer of a text-mode handle)

The error goes away if the first 'getChar' is commented out.

For libraries which are not yet using the Unicode I/O layer, having a way to temporarily set stdin to BinaryMode would be useful.