I've noticed difference in how maps behave when used as delegates between 2.5.12 (and earlier versions) and 2.5.13. Consider this code snippet:
In 2.5.x (up to 2.5.12) this code works just fine. Static field BAR is accessed from closure in line 10. But this breaks in 2.5.13. This affects also tests written in Spock. Utility method spock.lang.Specification::with works in the same way as callForAProblem.