I was having an issue with a JUnit plug-in test that I had just developed, throwing this exception:
java.lang.Exception: No runnable methods
The main reason for this error was that I was trying to run JUnit 4 tests on a JUnit 3 test case.
To resolve this, I found out I had a junit-4.5.jar that had been imported as part of the classpath, so I had to remove this.
I also had to edit the launch configuration manually to reset the test runner from JUnit 4 to JUnit 3.
(Also make sure you have the correct "required plugin" set, to org.junit or org.junit4.)