In reply to Al Evans:
So do these harsh (to my mind) comments apply to my mate whose stopper knot came undone too? Because he's certainly not mentally deficient, (I think his IQ comes in at about 160 as I remember, we chatted about IQ tests once) and he's not generally careless ... he just left too short a tail and the knot jiggled itself undone.
I think the reason people react with the 'oh he must be an idiot' type response is distancing ... ie they don't want to believe it could ever happen to them, because it's too scary.
The way to stop it happening isn't to slag off anyone it happens to. It's to pull your knot tight, add a stopper, again pulled tight, and make sure the tail is long enough. And if lapping, still check it each time.