The new routing feature on openstreetmap.org will make it a lot easier to find certain types of errors in the map. I tried it out and found errors that I would have overlooked otherwise.
Autoroute 40 is a limited-access divided highway in Quebec. Crossing gaps in the median strip are used for circulation during roadworks. These gaps are currently tagged as service roads on OSM.
What route should you take if you miss your highway exit and you want to go back in the opposite direction? If you are a boring law-abiding citizen, you will have to keep driving until you can take the next exit and then re-enter the highway in the other direction. OSM's routing algorithm, however, relying on less-than-perfect road access data, worries not about following such pointless traditions. Its free mind comes up with a more daring, if illegal, route that involves a hard 180-degree left turn through a crossing gap:
Clever, but a little too dangerous for my taste. Adding access=no to those service roads turns OSM back into a less audacious, but safer route planner.