Let’s say that someone, for whatever reason (say he forgot it was Behab), accepted upon himself to fast Behab but didn’t fast one of the days (say, for instance, he fasted Monday but not Thursday).
Does he have to make it up by fasting another day?
If so, does he make it up on a specific day (ex. the Thursday following the second Monday of Behab), or can he make it up on any weekday? Does he have to start over from the first Monday to complete a full set of Monday-Thursday-Monday?
If a full minyan managed to miss a day of Behab davening, do the above rules apply for them to make a minyan for Selichos?
Based on Shulchan Arukh OC 568:1-2, if someone forgot and ate on a fast day with a fixed date, he doesn't have to fast a different day instead, and the communal Bahab in Iyar and Marcheshvan is considered a fast day with a fixed date (as opposed to accepting on yourself to arbitrarily fast a personal Bahab some other time, where if you missed a day you could just restart the set some other time). So if he missed Thursday he still has to fast on Monday (and he should probably say Vidui for his mistake).