I went to help my sister move and was to be gone only a few hours. When I called to check on my Mom she said the dog had been throwing up a lot. I told her I would take care of it as soon as I got home. When I inquired about what food she gave the dog it was glucosamine tablets and the dosage was 2 a day. The dog ate way more of them as she poured the entire package that I had left on the shelf. The dog was very sick with throwing up, but otherwise ok. It just hit me kind of hard that my Mother is not the Mother that I used to know. How do you deal with this ? My Mother Years ago would never had made this mistake, yet she was trying to be helpful. She worried later that my dog would be mad at her for making her sick. It is like a charm od worried about stuff. Emotionally this is hard.
As Eyerishlass said above, it is so difficult to watch our parents age and decline. I am seeing my Dad  now going through this, and I am having a hard time dealing with as Dad was always so sharp.
No one ever thinks, "I'd better watch mom more closely so she doesn't give the dog it's medication instead of it's food." Usually a serious situation occurs and THEN we realize that we need to put safeguards in place. In other words, we don't realize that our parents need more assistance or supervision until a situation presents itself that demonstrates more assistance and supervision is necessary.
How to deal with this? It's like putting out fires and/or trying to stay 3 steps ahead so the fire doesn't start to begin with.