A Nature paper published in 2000 currently has around 400 citations, but there is a mistake in the paper and surprisingly, still it gets citations. The mistake affects the result of the paper in a way that half of the arguments in the paper are invalid.
I warned the authors two years ago and they confirmed the mistake. I expected them to put some announcement that there is a mistake in the paper to avoid misleading researchers, but unfortunately they have not done so.
How should we address these situations? Should we send a comment and report it to the editor? Is it rude? Or should we simply dismiss it because it is an old paper?