This is a replacement for my mixer I received as a gift in May. After the first use, I am having the same issue. The mixer will not turn off, unless I unplug it. I only made a bat h of oatmeal cookies. 
I like the mixer but needing to unplug it is ridiculous. They just don't make them like they used to. I feel if I exchange it again (which takes a month because we live in Hawaii) I will just get the same problem in another new mixer.
Disappointed.  I bake a lot.