Two types of mistakes can be made while blowing the shofar. The first type is when the shofar-blower intentionally blows a blast which turns out to be the wrong blast. In this case, he must return to the beginning of the set. The second type is when he tries and fails to blow the proper blast. In this case, he need not repeat the set. Rather, he should take a deep breath and blow the proper blast. We will now explain further.
The first type is when the shofar-blower got confused and blew a teru’a between a teki’a and a shevarim, or a shevarim between a teru’a and a teki’a. Since he intended to blow the blast that turned out to be incorrect in the middle of a set, he must go back to the beginning of the set. This is true even if he did not complete the blast, but only blew for the length of a tromit (SA 590:8). Likewise, if after completing a shevarim he took a breath and blew another shever, he must return to the first teki’a of the set. The additional shever was an alien blast in the middle of the set. Similarly, if after blowing a teru’a he took a breath and began another teru’a, he must return to the beginning of the set (SA 590:8).
The second type is when the shofar-blower tries to blow the last teki’a of the set, but what comes out is closer to a shevarim or a teru’a. He should take a breath and blow a proper teki’a. He need not go back to the beginning of the set, since he did not mean to blow the wrong blast. Similarly, if he was trying to blow a teru’a or a shevarim but ran out of breath in the middle, he must blow that blast again, but he need not return to the beginning of the set. Some shofar-blowers like to extend the last tromit of the teru’a. They must be careful when doing so, because if it sounds like a shevarim, the teru’a must be repeated (MB 590:31).
If the shofar-blower blew three tromitin of the teru’a but was unable to continue, and he paused long enough to take a breath, he must return to the beginning of the set. Since some maintain that three tromitin are sufficient to fulfill the obligation, if he blows the nine blasts of a teru’a after taking a breath, it would be as though he blew two teru’ot (AHS 590:20).
When there are two minyanim near each other, once the first minyan has started blowing the shofar, it is preferable for the second minyan not to start blowing until the first one has finished the initial thirty blasts. This is because some maintain that if a person hears other blasts in the middle of the shofar-blowing, even if he does not intend to fulfill his obligation through them, they invalidate the blasts he has already heard. Even though the halakha follows the view of most poskim, who reject this, it is preferable le-khatḥila to defer to those who are stringent (BHL 590:8 s.v. “ke-mitasek”).
. If, while blowing a set of tashrat, the shofar-blower blew the first two blasts of a shevarim and then began a teru’a, he must go back and blow a shevarim-teru’a again. However, he need not repeat the initial teki’a because even if he got confused (as opposed to being unable to blow the correct blasts), since he is still in the middle of the shevarim-teru’a, and the sounds of the teru’a are part of it, he is considered like someone who is unable to blow, rather than someone who made a mistake (SA 590:7; MB ad loc. 27-28).