My MIL is normally horrible so I would limit myself to things that make me wonder if she is OK in the brain department:
She introduced DS to her aquaintances with a different name, and when I asked why, she said that she didn't like his name and that she will continue to introduce him with the name that she had chosen for him.
She believes that oil paint is perfectly safe for toddlers and even edible because it's made with natural products (cadmium, yuuummy! ).
She thinks it's OK to serve and mix yesterday's o dog food with the one of the day with the same spoon she is using to feed DS, but find it "disgusting" not to wear shoes while you are at home.
She has some binoculars in her terrace to spy on the neighbours
IF DH's is using the phone, she picks up the other extension to hear the conversation
She walks through Harrods wearing a fluffy multicoloured rabbit skin jacket while loudly commenting about the "bad taste in dressing" of English people.
And the list goes on...