Claire Kramsch, at about makes the claim "I'm lovin it" of the McDonald's slogan is ungrammatical. (See link two posts below, about an hour into the video). I had no idea why she would make this claim. Then I looked it up.
It turns out that some grammar books claim that so-called "stative" verbs cannot be used in the progressive. No obviously this is false as a hard-and-fast rule of grammar. I was a little shocked that such an intelligent person would not realize that the situation is a lot more nuanced than that.
Of course, you can't just say "I am believing in God" when you mean to say "I believe in God." But you can say, "When did you stop believing in God?" or "I am remembering my dreams much more since I stopped drinking." You can use "loving" as a synonym for "enjoying," and in that case it can be used in the progressive. Waiters ask me how my food is tasting all the time. A stative can be used as a progressive to describe a more dynamic process. There is a big difference between "what does cilantro taste like?" and "how is the food tasting" [right now].
My basis for these claims are (1) My impeccable native speaker intuitions. (2)Evidence that such phrases are in common use. (3) The fact that you cannot change the progressive back into a straight present and maintain the same meaning. For example, "I love it" is not a substitute for "I'm loving it" in the slogan. "I love it" means I love it generally. "I love Indian food." "I'm loving this Indian food," means "I have been enjoying recently" or "I enjoy at the present moment, this Indian food."
A grammatical error is an actual mistake, when a native speaker slips up a bit. A nuanced use of a verbal mode to convey a different meaning is not a mistake.
The prestige grammar might not have caught up to the more dynamic use of the progressive with stative verbs. So much the worse for the prestige grammar.