If you are davening on Shabbos and accidentally start saying weekday brachos (ata chonen etc) you continue until the end of the bracha where you caught yourself and then start the Shabbos bracha. The question is if you passed Bareich Aleinu and said tal umatar. Do you go back and say the correct Bareich Aleinu before saying Shabbos bracha? I guess this would depend on the reason why we go back for tal umatar. If the reason is that there is some sort of “chiyuv” to daven for rain so therefore if you miss it you go back. But since on Shabbos we don’t have that “chiyuv” so you wouldn’t. This is my svara, I have no idea what the actual answer is.
A followup question is what if this happens on the 2nd day of Yom Tov. Is that already considered a time when you are supposed to say tal umatar? What if first day Chol HaMoed is Shabbos, is that the same as the 2nd day Yom Tov question or perhaps because it is already Chol HaMoed it is different. If you were to catch yourself in the middle of Bareich Aleinu, before tal umatar, on one of these days, do you say tal umatar even though the tzibur hasn’t started saying it yet? All very intriguing questions to keep us up all night. Keep up the good work PBA!