Jenkins 1.501 was running stable / no problem is log.
Then I added "JaCoCo" plugin from Update Center and restarted Jenkins.
When booting, Jenkins then scared me to death with a stack trace:

13.02.2013 09:53:32 hudson.ExtensionFinder$Sezpoz scout
WARNUNG: Failed to scout hudson.plugins.git.GitSCM$DescriptorImpl
java.lang.InstantiationException: file:/usr/share/tomcat6/.jenkins/plugins/git/WEB-INF/classes/META-INF/annotations/hudson.Extension might need to be rebuilt: java.lang.ClassNotFoundException: hudson.plugins.git.GitSCM$DescriptorImpl
at net.java.sezpoz.IndexItem.element(IndexItem.java:144)
at hudson.ExtensionFinder$Sezpoz.scout(ExtensionFinder.java:649)
at hudson.ClassicPluginStrategy.findComponents(ClassicPluginStrategy.java:312)
at hudson.ExtensionList.load(ExtensionList.java:295)
at hudson.ExtensionList.ensureLoaded(ExtensionList.java:248)
at hudson.ExtensionList.iterator(ExtensionList.java:138)
at hudson.ClassicPluginStrategy.findComponents(ClassicPluginStrategy.java:311)
at hudson.ExtensionList.load(ExtensionList.java:295)
at hudson.ExtensionList.ensureLoaded(ExtensionList.java:248)
at hudson.ExtensionList.get(ExtensionList.java:153)
at hudson.PluginManager$PluginUpdateMonitor.getInstance(PluginManager.java:1066)
at hudson.maven.PluginImpl.init(PluginImpl.java:54)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:616)
at hudson.init.InitializerFinder.invoke(InitializerFinder.java:120)
at hudson.init.InitializerFinder$TaskImpl.run(InitializerFinder.java:184)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:883)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:636)
Caused by: java.lang.ClassNotFoundException: hudson.plugins.git.GitSCM$DescriptorImpl
at hudson.PluginManager$UberClassLoader.findClass(PluginManager.java:942)
at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
at net.java.sezpoz.IndexItem.element(IndexItem.java:134)
... 24 more

Obviously it has a problem with the GIT plugin now – but I didn't touch that one, and it worked well before. Also, the GIT plugin seems to work fine, still.

As all works well, maybe the only bug is that the WARNING is actually printed?