I don't know why this problem appeared today, when running a job using Java 6, we got the following error:
JENKINS-18403 JDK 5 not supported to run Maven
And then Jenkins switched to Java 7 (which we don't want).
I checked the source code, and I guess a UnsupportedClassVersionError happened.
It would maybe best to log the exception when it does to at least ease diagnostic if the problem is coming from us… but I don't think it is the case because it happens really at the beginning of the execution…