Riots, Drills and the Devil: Part 1 - S1-E6
Corrected entry: Michael rescues Sara by crawling through a pipe leading into the infirmary. Later in the series Michael must find another way to get inside the infirmary, because the way he planned to go could not be used. So surely the prisoners could have used this way to get inside the infirmary. There is no reason why Michael could not have even thought of going this way given he had used it before. It would have been much easier and not as risky going this way rather than through the prison yard and corridors. This could have been Michael's plan to get inside the infirmary from the beginning.
Correction: Sara wasn't at the infirmary during the riots, but in sick bay: a different part of the prison complex in a whole other building.