A lot of parents are between a rock and a hard place, but a lot the people on here are being incredibly judgemental, instead of being grateful that life has so far permitted them to perfect parents.
There's a massive difference between a lapse of judgement/desperate situation, and the conscious choice to leave a four-year-old at home while you run an errand (and to continue doing this because, hey, nothing happened so obviously it's safe).
The whole 'oh, so you think you're PERFECT' crap has been done to death. As has the hyperbole about parents being 'between a rock and a hard place' and therefore immune from any sort of responsibility for their choices.
As MaQueen points out, almost every example given on this thread (and others) turns out to involve leaving a kid unsupervised for something that's absolutely not life-and-death: picking up an ingredient missing for dinner, collecting a package. It's a choice, usually dressed up in hyperbole and defensiveness to look like an urgent necessity ('I HAD to go, or we would have STARVED, is THAT what you want?').
Come on here and say 'I just smashed my last vial of insulin, and the pharmacy closes in five minutes, and I don't have a car and I've just moved to the area and don't know a single person who can help' -- that would be a more mixed discussion.
Not something which 99% of the time boils down to: 'fuck, I forgot the eggs, we'll all have to have toast for dinner and everyone will moan at me, so I'm just going to run out and leave my four-year-old at home alone'.