The debugger tries to handle unexpected strangeness in the program and is user input, but it is probably not perfect. Hence, it has a fallback mechanism that tells you when it has failed; it prints an error message like the following and exits.
Concert Debugger internal error: segmentation fault Please e-mail concert-bugs@red-herring.cs.uiuc.edu, enclosing this Emacs buffer and the Concert program being debugged. See the Concert Tutorial for how to send a Concert bug report. process Concert Debugger exited abnormally with code 666
You should report all such bugs to concert-bugs@red-herring using the GNATS bug reporting software that comes with the Concert System.