If you are running Typemock Isolator along with another profiler like NCover and a crash occurs (e.g., the parent build process gets killed), it has the potential to corrupt the registry. What that means is that subsequent operations when you link/unlink with your coverage profiler may not work properly.
For NCover, you may see the build fail with exit code #20000 and the message "NCover.Console is returning exit code #20000. NCover couldn't create a coverage report." The reason it couldn't create a coverage report is that Isolator and NCover weren't linked correctly so NCover wasn't actually running.
To fix it, repair your NCover installation. This will fix the corrupt registry keys and subsequent Typemock Isolator/NCover linkages will work correctly.
Thanks to Ohad at Typemock, Alan at NCover, and Jamie Cansdale from TestDriven.NET for helping me track this one down.