So far our GPS unit has never challenged our decision when we've elected to diverge from it's preferred route, other then a couple of attempts to persuade us to "round the block." It then gives up and mutters "recalculating."
Now as the good Capt. Morse might phrase it: "You lazy lubber, are you asleep? You've missed the channel! Swing wide, round the red nun over there (No, not Sinsull, the other nun). Oh, bugger (or substitute your favorite nautical profanity), now you've done it! Now we're hard aground!"