shalom-what is the halacha if one arrives at a minyan busy davening the shemonah esrah and one is unsure if they are davening mincha or maariv ,can one daven mincha in the same shul as the minyan ?this is the scenario for yomei chol ,please quote reference,if possible,toda rabah
You may daven Mincha even if the minyan is currently davening Maariv.
Pri Megadim O:C 236 and Mishna Brura s”k 11 indicate that this is permissible. This is also the opinion of Igros Moshe 3:37, Zachor L’Yitzchak Siman 70.
cf. Shu”t Haelef Lecha Shlomo [Rav Shlomo Kluger] O:C 94, who prohibits this practice.