When my solution gets reloaded because of external changes (e.g. due to a git operation), frequently my tests in the Unit Test Sessions window disappear. Sometimes all of them, sometimes just a few. When that happens, the only way to get them back is to restart VS. They still appear in Unit Test Explorer, but attempting to run from there has no effect.
VS 2013 (12.0.31101.0), Resharper 188.8.131.52 Update 1