I never do anything for a missed mile split. I have that location from my first ride. If I miss a mile split on my first ride I make extra special sure I don't miss it on my second.
If I go off course a few hundred counts I get off and walk my bike back. It always seems like it is really inaccurate when I do that, but I never seem to notice much difference in counts per mile for miles with our without a backup. It has certainly never caused me to be outside tolerance for the whole ride.
I like the turn-around and ride back technique mentioned by Winston for bigger screwups. But I have a GPS on my handlebars that shows me the correct route, so I can see within a few hundred counts that I'm not on it anymore. If I do end up going off course for while for some reason though, I'm going to use that technique. Thanks!