@AndreasS: Thanks for the definition. That's how I had understood it, but translations like 'decisive factor' suggested earlier had left me somewhat uncertain. Though I guess that would still work in some contexts where it was clear that the factor was decisively negative.
I didn't remember that particular discussion with Nancy (though I'd be glad to see Nancy around again), but it seems to be the second of these: related discussion:K.O. Kriterium related discussion:KO Kriterion related discussion:k/ko.html
I still have the feeling there's a more common way to say this in English, maybe a clause, but it's not coming to mind. 'The last straw' isn't really it. So far 'fatal flaw' seems to be the closest.
'Showstopper' would probably work in computer contexts, but I don't know about business or manufacturing or anything for a more general audience. At least, it was new to me, and since the original word actually has a positive connotation -- an act that stops the show because it brings so much applause -- it's not even all that straightforward to figure out from context.