The problem is that idiomatic Go reuses err for multiple calls. So if you already have one call and check err after, it counts as used, and forgetting to check it on subsequent calls is not flagged.

True. Not a big issue in practice and there are linters and whatnot. Variable shadowing can happen.

But it's a bit orthogonal of a concern. I do have things to say about errors but my complaints are a bit more nuanced and made with hindsight.