Breaking into the debugger

OK, so rails has detected an error (e.g. "can't convert Symbol into

Is there a way to break into the debugger without having to either

(a) insert a debugger statement into the source and rerunning the page?
(b) Set a breakpoint?

That is, is there a "break into the debugger on error" setting?

Netbeans 6.8 is supposed to have added conditional breakpoints and
"break on exception", but although the conditional breakpoints work
fine, I can't find any info on how the break on exception is supposed
to work. It certainly doesn't "just work" for me.

It's not a crisis, because if I need to I copy the filename/path from
the call-stack output in the browser, and open it in Netbeans, put a
breakpoint on the line and refresh...
Nine times out of ten that gets me where I need to be - but if you do
find out how to get the automatic breakout working, I'd love to know
(I'd love an "immediates" window too... but now I'm just waxing about
features that would be gravy... the debugging as it is is fantastic

PS You don't say what OS you're using, but I've had debugging running
in Netbeans on Windows and Linux (although I don't use Windows daily
any more...)