Thanks, it works perfectly now ! (Also, Ctrl+C on an empty buffer did output
a "command not found" message with the previous command run...)

I attached a branch to Micah's bug:
https://bugs.launchpad.net/ubuntu/+source/command-not-found/+bug/624565

2011/8/15 Bart Schaefer <schaefer@brasslantern.com>

> On Aug 12, 10:26pm, Micah Elliott wrote:
> } Subject: Re: Idiom for booleans
> }
> } > On Aug 12,  5:24pm, Micah Elliott wrote:
> } > }
> } > }   % (( 0 ))
> } > }   ((: command not found
> }
> } > This has to be something in your configuration.  Try it with zsh -f ?
> }
> }   . /etc/zsh_command_not_found
> }
> } That's a pretty handy utility (that I would expect to be popular), but
> } for now I'm going to live without it. I see it's a problem on the last
> } few Ubuntu releases, and I can't believe other people aren't seeing
> } the problem.
>
> I've had a Ubuntu laptop for a couple of years now and never noticed
> this utility (not that I'd have used it if I had) so it's not all that
> surprising to me that it isn't a widespread problem.
>
> Ironically, the very first line of the precmd function defined by
> /etc/zsh_command_not_found begins with
>
>   (($?)) && ...
>
> Obviously this implementation predates zsh introducing its own hook for
> command-not-found.  It could have been done correctly, though, by the
> simple expedient of
>
>   (($? == 127)) && ...
>