Well, of course it is unconscionable.
Some people are just dense. There are probably some less than honorable mistakes as has been suggested---we humans can act pretty wacko at times.
On the other hand, when our youngest boy was diagnosed with ADHD (at the same time I was); we subscribed to a monthly newsletter that had a number of articles and stories each issue.
One issue had the story of a mother who constantly would leave a medication that she had to take lying around---a dose that she intended to take, but got distracted, and left the pills on the table.
He husband frequently found them and talked to his wife about the danger of continuing to leave them around, but she was unable to deal with her ADHD in that area.
Subsequently, their toddler ingested them. It was a fatal mistake.
Unacceptable, of course, but how does one judge her?