Except the surprisingly common case that lanes are mislabelled... Ie. "I was in the lane marked M4, I followed that lane round perfectly, and suddenly the label in the lane changed to A205! I better change lanes. Oh actually the lane I was just in is now marked M4 again."

The problem seems to be that a lane that goes to two places is sometimes marked inconsistently with each of the two places - meaning you think you are in the wrong lane, when you are not.