Just like a b-

If you read the screenplay for The Dark Knight in the scene in Hong Kong before Batman goes into the big bank and grabs the crooked banker, Lucius Fox shows him a 3D map of the office created by sonar and you've got the following dialogue exchange:

WAYNE: (smiles) Sonar. Just like a b-

FOX: Submarine. Like a submarine.

But in the film the exchange is:

WAYNE: (smiles) Sonar. Just like a -

FOX: Submarine. Like a submarine.

When I watched the movie I never got the joke and knew it was a joke since they shared a little smile. But I sure got it once I read the script.

This is a good example of something that works great on the page, but doesn't work at all on screen. People don't stop mid-word and, if they did in the film, it'd sound really artificial and forced.

My guess is they found out it didn't sound right during the shoot and made a fix they hoped would work.