If you run jtreg with -noreport and files underneath more than one TEST.ROOT, you get:

jtreg -noreport -jdk:/home/martin/ws/jdk/test/../build/linux-x86_64-normal-server-release/images/jdk jdk/java/lang/Class/IsAnnotationType.java langtools/tools/javac/annotations/repeatingAnnotations/BasicRepeatingAnnotations.java
Test results: passed: 2
Error: Unexpected exception occurred! java.lang.IllegalStateException
java.lang.IllegalStateException
at com.sun.javatest.regtest.config.TestManager.getReportDirectory(TestManager.java:228)
at com.sun.javatest.regtest.report.RegressionReporter.report(RegressionReporter.java:123)
at com.sun.javatest.regtest.tool.Tool.run(Tool.java:1231)
at com.sun.javatest.regtest.tool.Tool.run(Tool.java:962)
at com.sun.javatest.regtest.tool.Tool.main(Tool.java:134)
at com.sun.javatest.regtest.Main.main(Main.java:58)

jtreg, version 4.2 dev b12