This story recently appeared in the Guardian newspaper, about a schoolboy who bullied Tesco into changing some 'ungrammatical' wording on its juice cartons. As you can probably tell, I'm (unusually) on Tesco's side here, or rather I would have been if they hadn't caved in instantly and completely.
Here's what happened: this 15-year-old boy noticed that his orange juice was described as being made with the 'most tastiest' oranges, and he felt that it should be either 'tastiest' or 'most tasty'. He felt this so strongly that he wrote first to Tesco customer services, and then to the Daily Mail when he didn't get a reply. And he wrote a real letter, with a stamp, not an email, which is how you know he was incandescent.
To clarify: yes, it 'should' be one of the two options he provided. The superlative doubling that he objects to is a very common feature of everyday speech, and I hear it approximately once a day (I listen out for it because I like it). In writing or formal speech it is considered wrong, and any piece of writing such as an essay should not include this construction. I'm very pleased that the standard of education in our schools is such that this child not only knows this, but cares about it. He sounds utterly insufferable, but I'm sure I was also insufferable at his age, so I'll let him off with that.
I do, however, want to suggest a better way for Tesco to have responded (rather than sending a grovelling letter promising to change the wording). The wording was probably carefully selected by a copy writer who knew exactly what they were doing, picking a construction that's frequent but not prescriptively correct, in order to come across as informal, friendly and possibly more eye-catching. Innocent, for example, while not using any 'ungrammatical' constructions that I can see, do their utmost to make their blurb informal, using no capital letters, lots of contractions ('we're', etc) and words like 'stuff', none of which would be acceptable in formal writing.
It's OK to use non-standard language if you know what you're doing and it's for a particular effect. As the saying goes, you have to know the rules before you know how to break them. Had Tesco been a linguist (and perhaps their copywriter is - lots of our students go on to do jobs like that) they might have responded to this young pedant with some facts about the frequency of use and the contexts in which superlative doubling is found, to demonstrate that it is not in fact ungrammatical, but merely register-specific. Then they could have explained to the young man that this wording was intentionally chosen to give the impression of a nice, friendly orange juice seller that you can trust, to mitigate the fact that you're buying concentrated orange juice from a huge corporation that probably pays its orange growers virtually nothing (I don't know this - just guessing).
Actually, the text from which this doubled superlative is taken is not specially informal, so it probably was an oversight. But there we go. I do think it's important not to always uphold the 'rules' of grammar, as being prissy about it is what causes people to dislike grammar when really it's such an interesting and fun thing, if you just look at it in the right way. I'm much more concerned about the genuinely ungrammatical things people (=students) write. If they'd never say it, why do they write it? But that's another rant for another day.