You Introduced Yourself in Japanese and Accidentally Called Yourself Sir.

Why this phrase

A first-meeting role-play. Hana, playing the Japanese person he is meeting, asks 「お名前は？」 and Kofuku, playing a traveller, answers 「ジョンさんです」; she has to keep a straight face before she fixes it. The rule is one line long: さん is a title you hand to other people, and you never attach it to your own name when you introduce yourself, so 「ジョンさんです」 lands like introducing yourself as Mister John. Keep that Mister John gloss in the dialogue. The fix is deleting one syllable. Hana says 「自分の名前には、さんを付けませんよ」, and Kofuku repeats it framed: 「はじめまして。ジョンです。よろしくお願いします。」