Shows up in the eclipse problem view as soon as the jar file is replaced on the build classpath.
Just tried it with joda-time 2.0 and same problem.
Perhaps version 2.x is built with different Java version? Not that up to speed with bytecode version issues viz Java/Scala.
Nothing in particular I need from joda-time 2.x at present, but would like to know if it could be a problem in future.