There is a very widespread custom of commencing one's recitation of the Shema with the three-word phrase, אל מלך נאמן, when davening alone, and of having the shaliach tzibbur repeat the last two words of the Shema, plus the first word of Emet veYatziv (= ד אלהיכם אמת) when davening with others (see Orach Chayim 61:3). The reason for this is that the Shema has exactly 245 words, and by adding an additional three we bring that number up to 248, which is the number of positive mitzvot (and human limbs, etc).
(See also Siddur Vilna, pp. 83–84 and 88)
My question: if somebody regularly davens alone, they might get into the habit of adding three words at the beginning, as described. In the event that s/he then finds themselves davening with others, but accidentally says אל מלך נאמן before commencing, what should s/he do?
- Should s/he continue as usual and not worry about it, since it's only a custom and does not affect the status of the Shema?
- Should s/he pause, then recommence without that additional passage?
- Should s/he continue as usual, but not wait for the shaliach tzibbur and just continue straight into Emet veYatziv in order that the number of words recited in the Shema is 248 and not (technically) 251?
- Or is there something else that should be done?