How is this possible?

I read this tragic story in the papers.  How is this possible?  It is obviously a tragic accident but dog owners know about dogs in cars.  Every dog owner does.  This makes me so mad.  We have had the coldest winter in decades and the first rays of sun come out and this is what happens.  Just think everybody out there.  Just think!  Be mindful.

“A school's 'therapy' dog has died after being left in a hot car for six hours because of a 'staff mix-up'. Ruby,  the Labrador was used to help lower-ability pupils at Sittingbourne Community College in Kent.

Owner Dave Carden, 53, a PE teacher, routinely took her in each day and left her in his car for another teacher to collect and take into class.
But on Tuesday - one of the hottest days of the year so far - she was not collected and remained in the car from 8am to 2.35pm.
Outside temperatures rose to the 20C mark and by the time Carden returned to his car, five year-old Ruby was dead.
He rushed her to the vets but there was nothing they could do to save her.”

RIP Ruby.