The class_terminate event can be used to create a global error handler without touching the iis config. When ASP encounters an error it server.execute's the configured 500 handler and then return to the orginal script to clean up all remaining objects.
This offers you an opportunity to create a global debugger object and use the class_terminate event to handle the errors (eg print out debug info).
Eg:
class cDebugger
private sub class_terminate
if err then
response.clear
dim asp_error
set asp_error = server.getLastError()
response.write asp_error.description
...
...
end if
end sub
end class
set [_debugger] = new cDebugger