Mishnah 4 in Maseches Shabbos deals with a case in which someone circumcises the wrong baby on the wrong day, and that day happens to be Shabbos.
The first case - circumcision before its time:
Someone has two babies: for one of them, the day of circumcision, the eighth day, falls after Shabbos - let us say on Sunday; the other is due to be circumcised on Shabbos. He erred or forgot, and on Shabbos he circumcised precisely the baby whose time was Sunday - one day too early, while the baby is not fit for circumcision until the eighth day. In this case he is "chayav" - obligated to bring a chatas offering for desecrating Shabbos, since he violated Shabbos by circumcising the wrong baby, and he did not even fulfill the mitzvah of circumcision through it: a circumcision performed before the eighth day is not a fulfillment of the mitzvah of circumcision.
The second case - circumcision not at its proper time:
One baby is due to be circumcised on Friday, and one baby is due to be circumcised on Shabbos. He forgot the halachah, and on Shabbos itself he circumcised the baby whose time was Friday. True, the mitzvah is to circumcise on the eighth day, but from then on, whenever it is performed, the circumcision is valid - so what he did here was a circumcision after the eighth day, a 'circumcision not at its proper time.' Such a circumcision does not override Shabbos, and he did in fact circumcise on Shabbos.
Therefore it says that he is "chayav" - liable, because he transgressed a prohibition of Shabbos: the only time it is permitted to circumcise on Shabbos is for a 'circumcision at its proper time,' on the eighth day, and not for a 'circumcision not at its proper time' - which in this case was performed when the baby was actually in his ninth day.
The view of Rabbi Yehoshua:
Rabbi Yehoshua disagrees and says that he is "patur" - exempt from a chatas. His reasoning: one who errs in the course of performing a mitzvah, even when a desecration of Shabbos is involved, and who does fulfill the mitzvah through his act - is not liable to a chatas. And here the mitzvah of circumcision was indeed fulfilled, for the baby whose time was Friday, although it is forbidden to circumcise him on Shabbos, if one did in fact circumcise him on Shabbos, that is a fulfillment of the mitzvah of circumcision, since the child is already obligated in circumcision at that time. According to Rabbi Yehoshua, one who fulfills the mitzvah in the course of desecrating Shabbos need not bring a chatas offering.
In summary: this Mishnah presents two cases of error in circumcision on Shabbos - a circumcision before its time (the baby whose time was Sunday), and a circumcision not at its proper time (the baby whose time was Friday). In both cases the Tanna Kamma holds that he is liable to a chatas for desecrating Shabbos, while Rabbi Yehoshua exempts him in the second case, because one who errs in the course of performing a mitzvah - and actually fulfills it - is exempt from the chatas.