If one had 'pas habah bekisnin' thinking that he needs to make a mezonos followed by al hamichya, and after doing so discovers that he should have washed, said hamotzi and then birkas hamazon, what should be done now after he has said al hamichya?
There is much discussion about this topic by many Acharonim. Rabbi Braun discusses it here and more in detail in the Hebrew mareh mekomos on that halacha. The consensus is that one is not Yotzei and needs to bentch again as discussed in that source at length with many sources.
There is more debate about whether the obligation to repeat bentching remains Medooraisa or becomes Medarabun (meaning that one doesn't even fufill his Torah obligation with al hamichya. See there for sources.)*
The Rebbe in a letter to an individual who asked the same question states that the reason for the need to repeat bentching is because the 'ברכה מעין ג does not include ברית ותורה. (See Rabbi Braun's discussion on this and his other reason he brings down.)
*See for example Sharei Tzion to the Mishnah Berurah Siman 168 note 71.