Suppose someone is praying maariv, shacharis, or mincha on Shabas, starts the weekday amida's blessings, and then realizes, amid those weekday blessings, that he should be saying the amida of Shabas. As ruled in Orach Chayim 268:2, he should continue through to the end of whatever blessing he's amid, and then start the Shabas blessing. The reason he continues with the blessing he's amid, Mishna B'rura 2 writes, is that that blessing is really suitable for Shabas in theory.
Now, during the third weekday-specific blessing ("S'lach lanu"), it's customary to strike one's chest with his fist. Suppose someone realizes, after starting that blessing but before reaching the part where he strikes his chest, that he should be saying the amida of Shabas. Then, per the preceding paragraph, he should finish "S'lach lanu". I wonder whether he should strike his chest. Arguably, yes, why not? On the other hand, though, (a) maybe, just as we don't (usually) say viduy on Shabas, there's some reason not to strike the chest on Shabas, and (b) maybe in order to save oneself the shame of revealing he started the weekday blessings, he can omit striking his chest. Does anyone have any sources on this, please?