I've seen it happen.
They're obviously not doing it properly.
I visited a cider farm the other week, and they make jam there too, and the guy in there said that there's a certain amount of sugar you have to put in before it won't go off. They make sure they put enough in, at least, even if Tesco think they can economise on it.
( Moon Girl Technologies horrendous beanbag, Tue 12 May 2009, 16:04, archived)

I have that Bonne Maman stuff, maybe they're just as bad.
Actually, my nan's jam never went off.

AHHHH.
( Captain Wow currently being a cunt in Infamous, Tue 12 May 2009, 16:05, archived)