If View White Space is turned on the tab character (->) will appear above the coverage marker and will take interaction from it.
This makes the coverage marker really hard to click, you can only click on it at the edges.
If your text goes to the far left then normal characters don't take interaction from the coverage marker.
Edit, Advanced, View White Space.
Tested in VS 2010 and 2012 RC.
Running Resharper and NCrunch (Both Latest).