While walking in France I noticed that some paths appeared double and thought: I'll correct this when I return.
When looking in the OSM editor I noticed that one path is a track, while the orther is an administrative boundary of level 8.
On the web it also looks confusing:
http://www.openstreetmap.org/?lat=49.335734&lon=6.342121&zoom=18&layers=MBut I think that for interactive use this information is rarely needed, and thus better omitted.
The same applies in my opinion to power lines, by the way. The problem is that these cross the important information (roads and paths) and there are only so many clearly distinguishable rendering styles to choose from. Buildings do not have this problem usually.