If $ZTRAP contains invalid source code, GT.M displays an error message and puts the process into Direct Mode.

If the action specified by $ZTRAP results in another run-time error before changing the value of $ZTRAP, it may result in a loop that iteratively invokes $ZTRAP until a stack overflow terminates the GT.M image. Keep $ZTRAP simple and take special care to debug exception handling.