If one were to miss retzei in shalosh seudos or al hamichya on Shabbos, does one need to repeat the blessing?
Answer:
If one omits an addition, such as retzei, one needs to repeat the blessing only if the meal that the blessing is made for is obligatory. Because some opinions maintain that it is not obligatory to eat bread in the third meal on Shabbos, and there is likewise no obligation of eating cakes or cookies outside meals, one would not reapeat the blessing if one forgot to say retzei for shalosh seudos or the addition for al hamichyah.
Sources: Mishnah Berurah 188:26; Shaarei Teshuvah 291:1; Mishnah Berurah 291:6
