Lisp help: debug-on-error not effective twice?

[Lars Brinkhoff]

If I run this (in *scratch*) in GNU Emacs 21.3:

    (defun my-debug (&rest args)
      (print "foo")
      (throw 'error nil))

    (let ((debug-on-error t)
          (debug-on-quit t)
          (debug-on-signal t)
          (debug-ignored-errors nil)
          (debugger 'my-debug))
      (catch 'error
        (car))
      ;;(debug)
      (print (format "debug-on-error = %s" debug-on-error))
      (catch 'error
        (car)))

I get one "foo" from my-debug, then "debug-on-error = t", and then the
error "Wrong number of arguments: #<subr car>, 0".

Why isn't my-debug called twice?

If I uncomment (debug) and enter 'c' in the debugger to contine,
my-debug does get called twice.  Is debug doing something magic?

-- 
Lars Brinkhoff,         Services for Unix, Linux, GCC, HTTP
Brinkhoff Consulting    http://www.brinkhoff.se/