Hi Richard,
I don't know what could be the cause here without a proper bug report. But when I was using Eclipse and had this combination (Eclipse + Maven), I remember having trouble running tests in maven.
The reason is that the Eclipse compiler does things a bit differently than the JDK compiler, as Eclipse compiler is constantly running it overwrites classes compiled by javac, and when the tests run it might lead to inconsistent/incompatible bytecode. The only workaround was to terminate Eclipse and do a `mvn clean test`.
Hope that helps.
Cheers