Towards the end when the boys are trying to get into the school with a vaccination all of them are there except for Kenny and none of them are wearing their masks. Right as Mr Garrison shows up and the air Israel plane gets there, there is a shot of everyone including all the adults waiting for the vaccination outside of the school and you can see not just all four of the boys but they are all wearing their masks and then it cuts back and forth a few times and then all of a sudden it's back to how it was with just the three boys, no masks, no Kenny.
When Cartman is going over the first page of his 'sharing Kenny' options he states that he would have Kenny on Thursday/Friday. The chart, though, showed he would have Kenny on Wednesday/Thursday.