We've got an old mac slave. After a recent upgrade of Jenkins I was unable to start a slave on this slave. It seems like the Jenkins slave is trying to load an incompatible class using the 1.5 jre.

[07/17/13 15:16:33] [SSH] Checking java version of java
[07/17/13 15:16:33] [SSH] java -version returned 1.5.0_13.
[07/17/13 15:16:33] [SSH] Starting sftp client.
[07/17/13 15:16:33] [SSH] Copying latest slave.jar...
[07/17/13 15:16:33] [SSH] Copied 326,561 bytes.
Expanded the channel window size to 4MB
[07/17/13 15:16:33] [SSH] Starting slave process: cd '/hudson' && java -jar slave.jar
<===[JENKINS REMOTING CAPACITY]===>channel started
java.io.IOException: Remote call on mac failed
at hudson.remoting.Channel.call(Channel.java:722)
at hudson.slaves.SlaveComputer.setChannel(SlaveComputer.java:451)
at hudson.slaves.SlaveComputer.setChannel(SlaveComputer.java:347)
at hudson.plugins.sshslaves.SSHLauncher.startSlave(SSHLauncher.java:695)
at hudson.plugins.sshslaves.SSHLauncher.launch(SSHLauncher.java:489)
at hudson.slaves.SlaveComputer$1.call(SlaveComputer.java:230)
at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassFormatError: Failed to load hudson.slaves.SlaveComputer$SlaveVersion
at hudson.remoting.RemoteClassLoader.loadClassFile(RemoteClassLoader.java:291)
at hudson.remoting.RemoteClassLoader.findClass(RemoteClassLoader.java:229)
at java.lang.ClassLoader.loadClass(ClassLoader.java:316)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:374)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:242)
at hudson.remoting.MultiClassLoaderSerializer$Input.resolveClass(MultiClassLoaderSerializer.java:113)
at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1544)
at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1466)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1699)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1305)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:348)
at hudson.remoting.UserRequest.deserialize(UserRequest.java:182)
at hudson.remoting.UserRequest.perform(UserRequest.java:98)
at hudson.remoting.UserRequest.perform(UserRequest.java:48)
at hudson.remoting.Request$2.run(Request.java:326)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:72)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:269)
at java.util.concurrent.FutureTask.run(FutureTask.java:123)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:650)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:675)
at java.lang.Thread.run(Thread.java:613)
Caused by: java.lang.UnsupportedClassVersionError: Bad version number in .class file
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:675)
at java.lang.ClassLoader.defineClass(ClassLoader.java:520)
at hudson.remoting.RemoteClassLoader.loadClassFile(RemoteClassLoader.java:289)
... 22 more