I got the following errors after a project build was triggered:

java.lang.ClassFormatError: Truncated class file
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClassCond(ClassLoader.java:637)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:621)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:471)
	at org.jboss.tools.arquillian.core.internal.classpath.ArquillianClassLoader.findClass(ArquillianClassLoader.java:212)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:249)
	at org.jboss.tools.arquillian.core.internal.util.ArquillianSearchEngine.createArchive(ArquillianSearchEngine.java:740)
	at org.jboss.tools.arquillian.core.internal.util.ArquillianSearchEngine.getDeploymentArchives(ArquillianSearchEngine.java:704)
	at org.jboss.tools.arquillian.core.internal.compiler.ArquillianNameEnvironment.setEnvironment(ArquillianNameEnvironment.java:411)
	at org.jboss.tools.arquillian.core.internal.compiler.ArquillianCompilationParticipant.buildFinished(ArquillianCompilationParticipant.java:149)
	at org.eclipse.jdt.internal.core.builder.JavaBuilder.build(JavaBuilder.java:233)
	at org.eclipse.core.internal.events.BuildManager$2.run(BuildManager.java:733)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
	at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:206)
	at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:246)
	at org.eclipse.core.internal.events.BuildManager$1.run(BuildManager.java:299)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
	at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:302)
	at org.eclipse.core.internal.events.BuildManager.basicBuildLoop(BuildManager.java:358)
	at org.eclipse.core.internal.events.BuildManager.build(BuildManager.java:381)
	at org.eclipse.core.internal.events.AutoBuildJob.doBuild(AutoBuildJob.java:143)
	at org.eclipse.core.internal.events.AutoBuildJob.run(AutoBuildJob.java:241)
	at org.eclipse.core.internal.jobs.Worker.run(Worker.java:53)

and 

org.eclipse.core.internal.resources.ResourceException: Resource is out of sync with the file system: '/conference-schedule/target/test-classes/com/example/conferenceschedule/rest/SessionEndpointTest.class'.
at org.eclipse.core.internal.localstore.FileSystemResourceManager.read(FileSystemResourceManager.java:793)
at org.eclipse.core.internal.resources.File.getContents(File.java:289)
at org.eclipse.core.internal.resources.File.getContents(File.java:278)
at org.jboss.tools.arquillian.core.internal.classpath.ArquillianClassLoader.loadBytes(ArquillianClassLoader.java:321)
at org.jboss.tools.arquillian.core.internal.classpath.ArquillianClassLoader.findClass(ArquillianClassLoader.java:210)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:249)
at org.jboss.tools.arquillian.core.internal.util.ArquillianSearchEngine.createArchive(ArquillianSearchEngine.java:740)
at org.jboss.tools.arquillian.core.internal.util.ArquillianSearchEngine.getDeploymentArchives(ArquillianSearchEngine.java:704)
at org.jboss.tools.arquillian.core.internal.compiler.ArquillianNameEnvironment.setEnvironment(ArquillianNameEnvironment.java:411)
at org.jboss.tools.arquillian.core.internal.compiler.ArquillianCompilationParticipant.buildFinished(ArquillianCompilationParticipant.java:149)
at org.eclipse.jdt.internal.core.builder.JavaBuilder.build(JavaBuilder.java:233)
at org.eclipse.core.internal.events.BuildManager$2.run(BuildManager.java:733)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:206)
at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:246)
at org.eclipse.core.internal.events.BuildManager$1.run(BuildManager.java:299)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:302)
at org.eclipse.core.internal.events.BuildManager.basicBuildLoop(BuildManager.java:358)
at org.eclipse.core.internal.events.BuildManager.build(BuildManager.java:381)
at org.eclipse.core.internal.events.AutoBuildJob.doBuild(AutoBuildJob.java:143)
at org.eclipse.core.internal.events.AutoBuildJob.run(AutoBuildJob.java:241)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:53)

The project had been cleaned/built with maven from command line before, which should explain that some resources where out-of-sync.