Do you consider double negative to be a grammatical error?

Yes. I do consider it as a grammatical error. E.g. a negative statement containing two negative elements (for example he didn't say nothing ). You shouldn't use as it is given above. It can be 1. He didn't say anything or 2. He said nothing. So the double negative is considered as a grammatical error. I hope I have answered your question correctly.

Yes I do. Even if many people think that double negative in sentence gives a positive meanting, for ex.

- That won’t do you no good.

I would still simply say it with

- That won't do you any harm.

In official English laguage, double negatives are still considered as grammatical error. Dont you agree?