Botulism isn't a problem as long as the incipient yoghurt is exposed to oxygen and contains plenty of other micro-organisms: c. botulinum is anærobic and easily

Botulism isn't a problem as long as the incipient yoghurt is exposed to oxygen and contains plenty of other micro-organisms: c. botulinum is anærobic and easily out-competed by lactobacillus. There are spores of c. botulinum everywhere, but they don't normally cause a problem unless there's a deoxygenated environment (e.g. in the core of a sausage — Latin botulus, hence the name). The trouble with poorly-canned goods is that they are not exposed to oxygen, other micro-organisms have been killed off, but c. botulinum has survived: that's a perfect breeding ground for the stuff.