I davened at the early minyan today. After the later minyan (which was already under way by the time we finished Avinu Malkeinu) there is a bris scheduled. Therefore, we did not say tachanun, as per https://www.ou.org/torah/halacha/hashoneh-halachos/mon_02_06_12/ :
22:6 Tachanun is not said in a shul where a bris is about to be held
[this is, of course, following opinions that say that the father, sandak or mohel don't have to be in the room for the bris to affect an earlier minyan -- see here for sources]
What if, God forbid, it was discovered right before the bris that the baby was sick and the mohel decided not to perform the bris. Would those who davened at the earlier minyan "owe" a tachanun? Would it matter if the decision was made before the earlier minyan was finished so everyone was still "in" davening, as opposed to if they went home? Or is the fact that we have days when tachanun is skipped a reason not to feel that tachanun is important to make up when we wrongly leave it out?
This site says that one who skipped parts of davening in order to catch up makes them up afterwards - is that analogous to the case of the intentionally omitted tachanun which later becomes "not-omittable"?