For example, see this way: http://www.openstreetmap.org/browse/way/205859936
It was created with Potlach2, and it overlaps itself. The user created a way, added some nodes to the left, went back along the same nodes, and then continued the original way.
JOSM gives a warning for such ways.
They are hard to correct when such overlaps were done several times, e.g. in residential areas.
I’d like to see some warning to the user when he creates such ways, or better even not let him upload such data! (I know, even JOSM allows uploading such errors.)