Pesachim 72 - Circumcision and Passover on Shabbat - what's the difference?

Similar to the Passover sacrifice, if the circumcision day (eighth after birth) occurs on Shabbat, the circumcision is performed even though by doing it, one creates a wound. 

Let's say one had two babies that he needed to circumcise, one on Friday and the other one on Shabbat. He messed up and circumcised the Friday baby on Shabbat. He violated Shabbat but did the mitzvah. You can say that he was negligent. So why does Rabbi Yehoshua exempt him here but obligates him in a sin-offering in the Passover offering case that we saw earlier?

The answer is in the additional details. First, he circumcised the Friday baby. So he was still preoccupied with the remaining Shabbat baby. This explains his error and exempts him from the sin-offering. In the case of Passover, he already did the mitzvah the wrong way and had no other mitzvah to distract him, so there Rabbi Yehoshua makes him liable.

Art: Two Sleeping Children by Peter Paul Rubens