Slips of the Tongue/Finger

Here's an interesting article at Language Log, commenting on the (apparent) fact that slips of the tongue don't cross word-category boundaries ("She brought home a dog" might slip into "She brought home a duck" - or "a dock" - but not into "She brought home a drown"; nouns for nouns, not verbs for nouns), but writing/typing errors do cross the boundaries ("a corporate lawyer from The Down Chemical Company"). No one seems to have studied the reasons for this.