I think most parents, especially parents with 3+ kids, can look back at some instance of negligence in the raising of their kids that could have ended badly.
I once lost track of an 8 year old at Disney World. That could have ended badly and I'm sure I would have been judged as a bad parent by some but I would have also been judged if I had her on a leash.
I'm not equating this to forgetting a child in a locked car but I don't feel it's my place to judge someone that has this happen. My guess is that what they have to deal with for the rest of their life is brutal.
My kids are now 23, 22, and 19 and I'm thankful for getting them through their childhood relatively safely.
Discussions like this are very helpful for parents of small children because its just another reminder of how diligent you have to be to be a parent in this busy world in which we live.