When concurrently starting a REXX script from java, BSF4ooRexx seems prone to crashing (taking the JVM down with it).
See the attached test program to reproduce and a crash report. It doesn't consistently crash 100% of the time, but usually it does. Raising the THREADS constant seems to increase the likelyhood of the crash.
The vulnerability seems to be mainly while starting the REXX script: If it does start successfully, it seems to keep running as it should.