Everything has errors. The mentioned routing via hiking trails is no joke. I've been doing 20-miles detours from recommended routes more than once.
Than updating the data is another story. The Street Atlas 2009 doesn't include freeways that have been build after 1998. Driving with GPS in California, that has good number of new roads and developments can be quite frustrating.
Can you imagine driving MB on new road, while the screen is showing you on the old road 1/2 mile away?
OK, I did not consider it an error if the map data is not fully up-to-date. It cannot include roads that have been built and planned before the data was released (my map disk covers some roads that are being built, if does not direct you to those roads though).
I could think of calling it an error when a missing road has existed "long enough" before the map data was released. Again, I don't have experience from my neighbourhood but I remember some UK members talking about some well known roads missing quite some time, which is amazing. This however should not be an MB specific case but should concern all devices that use the same map data supplier.