Given this code:

module Main
    where

import Data.Word
import Unsafe.Coerce
import System.IO

nanFloat :: Float
nanFloat = unsafeCoerce (maxBound :: Word32)

float2word32 :: Float -> Word32
float2word32 = unsafeCoerce

nanDouble :: Double
nanDouble = unsafeCoerce (maxBound :: Word64)

double2word64 :: Double -> Word64
double2word64 = unsafeCoerce

main = do
    putStrLn $ "nanFloat  = " ++ show (float2word32 nanFloat)
    putStrLn $ "nanFloat  = " ++ show (float2word32 $ nanFloat + 1)
    putStrLn $ "nanDouble = " ++ show (double2word64 nanDouble)
    putStrLn $ "nanDouble = " ++ show (double2word64 $ nanDouble + 1)

If we compile with GHC and run, we correctly output:

nanFloat  = 4294967295
nanFloat  = 4294967295
nanDouble = 18446744073709551615
nanDouble = 18446744073709551615

But if we instead load in ghci, we get the following output:

nanFloat  = 4294967295
nanFloat  = 140247862083583
nanDouble = 18446744073709551615
nanDouble = 18446744073709551615

For some reason, ghci is displaying (nanFloat+1) as having significantly more digits than can possibly stored in a Word32 value.

Test system: Intel Core 2 Duo running Debian with GHC 7.8.2