With following specs:

package demo.test

import org.specs.Specification
import org.specs.runner._
import org.specs.matcher._

class SimpleUnitTest extends JUnit4(SimpleSpec)

object SimpleSpec extends Specification {
"This test" should {
"be run" in {1 must_== 1}
}
}

pressing ctrl-F11 causes the following exception three times in the log. It does not prevent the test from launching, but it certainly seems to slow it down.

org.eclipse.jdt.internal.compiler.problem.AbortCompilation: Pb(324) The type scala.collection.Seq cannot be resolved. It is indirectly referenced from required .class files
at org.eclipse.jdt.internal.compiler.problem.ProblemHandler.handle(ProblemHandler.java:121)
at org.eclipse.jdt.internal.compiler.problem.ProblemHandler.handle(ProblemHandler.java:179)
at org.eclipse.jdt.internal.compiler.problem.ProblemReporter.handle(ProblemReporter.java:1950)
at org.eclipse.jdt.internal.compiler.problem.ProblemReporter.isClassPathCorrect(ProblemReporter.java:3836)
at org.eclipse.jdt.internal.compiler.lookup.UnresolvedReferenceBinding.resolve(UnresolvedReferenceBinding.java:54)
at org.eclipse.jdt.internal.compiler.lookup.BinaryTypeBinding.resolveType(BinaryTypeBinding.java:122)
at org.eclipse.jdt.internal.compiler.lookup.PackageBinding.getTypeOrPackage(PackageBinding.java:168)
at org.eclipse.jdt.internal.compiler.lookup.Scope.getPackage(Scope.java:2284)
at org.eclipse.jdt.internal.compiler.ast.QualifiedTypeReference.getTypeBinding(QualifiedTypeReference.java:69)
at org.eclipse.jdt.internal.compiler.ast.TypeReference.internalResolveType(TypeReference.java:130)
at org.eclipse.jdt.internal.compiler.ast.TypeReference.resolveType(TypeReference.java:197)
at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.resolveTypesFor(SourceTypeBinding.java:1370)
at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.methods(SourceTypeBinding.java:1090)
at org.eclipse.jdt.internal.compiler.lookup.ReferenceBinding.availableMethods(ReferenceBinding.java:173)
at org.eclipse.jdt.core.dom.TypeBinding.getDeclaredMethods(TypeBinding.java:261)
at org.eclipse.jdt.internal.junit.launcher.JUnit4TestFinder$Annotation.annotatesAtLeastOneMethod(JUnit4TestFinder.java:97)
at org.eclipse.jdt.internal.junit.launcher.JUnit4TestFinder.isTest(JUnit4TestFinder.java:254)
at org.eclipse.jdt.internal.junit.launcher.JUnit4TestFinder.internalIsTest(JUnit4TestFinder.java:237)
at org.eclipse.jdt.internal.junit.launcher.JUnit4TestFinder.isTest(JUnit4TestFinder.java:200)
at org.eclipse.jdt.internal.junit.util.TestSearchEngine.isTestOrTestSuite(TestSearchEngine.java:58)
at org.eclipse.jdt.internal.junit.JUnitPropertyTester.isJUnitTest(JUnitPropertyTester.java:112)
at org.eclipse.jdt.internal.junit.JUnitPropertyTester.canLaunchAsJUnitTest(JUnitPropertyTester.java:87)
at org.eclipse.jdt.internal.junit.JUnitPropertyTester.test(JUnitPropertyTester.java:70)
at org.eclipse.core.internal.expressions.Property.test(Property.java:58)
at org.eclipse.core.internal.expressions.TestExpression.evaluate(TestExpression.java:99)
at org.eclipse.core.internal.expressions.CompositeExpression.evaluateAnd(CompositeExpression.java:53)
at org.eclipse.core.internal.expressions.AdaptExpression.evaluate(AdaptExpression.java:91)
at org.eclipse.core.internal.expressions.CompositeExpression.evaluateAnd(CompositeExpression.java:53)
at org.eclipse.core.internal.expressions.IterateExpression.evaluate(IterateExpression.java:150)
at org.eclipse.core.internal.expressions.CompositeExpression.evaluateAnd(CompositeExpression.java:53)
at org.eclipse.core.internal.expressions.WithExpression.evaluate(WithExpression.java:72)
at org.eclipse.core.internal.expressions.CompositeExpression.evaluateAnd(CompositeExpression.java:53)
at org.eclipse.core.internal.expressions.EnablementExpression.evaluate(EnablementExpression.java:53)
at org.eclipse.debug.internal.ui.launchConfigurations.LaunchShortcutExtension.evalEnablementExpression(LaunchShortcutExtension.java:287)
at org.eclipse.debug.internal.ui.launchConfigurations.LaunchConfigurationManager.getApplicableConfigurationTypes(LaunchConfigurationManager.java:742)
at org.eclipse.debug.internal.ui.launchConfigurations.LaunchConfigurationManager.getApplicableLaunchConfigurations(LaunchConfigurationManager.java:778)
at org.eclipse.debug.internal.ui.contextlaunching.LaunchingResourceManager.getParticipatingLaunchConfigurations(LaunchingResourceManager.java:492)
at org.eclipse.debug.internal.ui.contextlaunching.ContextRunner.selectAndLaunch(ContextRunner.java:127)
at org.eclipse.debug.internal.ui.contextlaunching.ContextRunner.launch(ContextRunner.java:81)
at org.eclipse.debug.internal.ui.actions.RelaunchLastAction.run(RelaunchLastAction.java:77)
at org.eclipse.ui.internal.handlers.ActionDelegateHandlerProxy.execute(ActionDelegateHandlerProxy.java:289)
at org.eclipse.core.commands.Command.executeWithChecks(Command.java:476)
at org.eclipse.core.commands.ParameterizedCommand.executeWithChecks(ParameterizedCommand.java:508)
at org.eclipse.ui.internal.handlers.HandlerService.executeCommand(HandlerService.java:169)
at org.eclipse.ui.internal.keys.WorkbenchKeyboard.executeCommand(WorkbenchKeyboard.java:470)
at org.eclipse.ui.internal.keys.WorkbenchKeyboard.press(WorkbenchKeyboard.java:824)
at org.eclipse.ui.internal.keys.WorkbenchKeyboard.processKeyEvent(WorkbenchKeyboard.java:880)
at org.eclipse.ui.internal.keys.WorkbenchKeyboard.filterKeySequenceBindings(WorkbenchKeyboard.java:569)
at org.eclipse.ui.internal.keys.WorkbenchKeyboard.access$3(WorkbenchKeyboard.java:510)
at org.eclipse.ui.internal.keys.WorkbenchKeyboard$KeyDownFilter.handleEvent(WorkbenchKeyboard.java:125)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Display.filterEvent(Display.java:1208)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1002)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1027)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1012)
at org.eclipse.swt.widgets.Widget.sendKeyEvent(Widget.java:1040)
at org.eclipse.swt.widgets.Widget.sendKeyEvent(Widget.java:1036)
at org.eclipse.swt.widgets.Widget.wmKeyDown(Widget.java:1669)
at org.eclipse.swt.widgets.Control.WM_KEYDOWN(Control.java:4281)
at org.eclipse.swt.widgets.Control.windowProc(Control.java:3977)
at org.eclipse.swt.widgets.Canvas.windowProc(Canvas.java:342)
at org.eclipse.swt.widgets.Display.windowProc(Display.java:4632)
at org.eclipse.swt.internal.win32.OS.DispatchMessageW(Native Method)
at org.eclipse.swt.internal.win32.OS.DispatchMessage(OS.java:2411)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3501)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2405)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2369)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2221)
at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:500)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:493)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:113)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:194)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:368)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:559)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:514)
at org.eclipse.equinox.launcher.Main.run(Main.java:1311)
at org.eclipse.equinox.launcher.Main.main(Main.java:1287)