If it tells you to drive into a lake, yes, you should doubt it.
http://www.independent.co.uk/news/world/americas/woman-follo...
> A woman following her car's sat nav on a foggy night took a wrong turn and ended up driving into a lake. [...] she lost her way and drove down a steep boat launch into Lake Huron. [...] Local police said the way the harbour’s boat launch is built means a wrong turn taken on a rainy night would come as a shock.
That is to say, when she drove into the lake it seems to have been because she wasn't following the directions, and this, combined with the fog and a confusing design for the boat launch, caused her to not realize that she wasn't on a regular road (albeit the wrong one) until it disappeared out from under her.
The article then concludes with some statistics about GPS causing distracted driving, without either connecting them back to the news item at hand or comparing them to distracted driving caused by non-electronic navigation.