DietersMistakesIn2021/Solution 58

Best  

is an example of surround in sente which is one of the patterns in this mistake analysis series. If elsewhere, then is big. Black can live but takes 15 points in sente because of a.

Sente  

Compared to answering locally, the difference goes up to more than 20 points, so that can be considered sente.

Mistake  

is premature. Black can break through the lines and there's bad aji at the marked point.