Revealing mistake: It seems that in the first shot the doctor injects something into the vaccine vial rather than extracting from it since the syringe plunger is moving inwards instead of outwards. In the next shot one can see that the syringe is being filled with the vaccine, however the arrangement of doctor's fingers is suitable for injection, which strongly suggests that the scene is played backwards.
Revealing mistake: The establishing shot of Boston General Hospital at the end of the episode shows the same building that was used for the BYU Medical Center in Provo, Utah eight episodes earlier in "Roadrunners" (note the vegetation under the part of the building that extends over the curb and the lettering on the building that says "Entry A"). The actual building is presumably in Kansas since the Kansas state flag was visible outside the building in the aforementioned episode.
Continuity mistake: In this episode the character of Billy Underwood was played by twin brothers Kyle and Ryan Pepi. If you pay attention you can tell when one twin was being used instead of the other as one of them has two freckles on his neck and another two by his right eye whereas the other does not.
Add timeSerious B
Continuity mistake: When the Lone Gunmen speak with Susanne Modeski, in the background there are some cars parked in front of the Stadium Inn. However, just seconds after, when the telephone rings, there is a white truck parked at the entrance. Later on, the cars are back again.
Factual error: In the background the full moon is visible and it's early morning (approx. 6:00 AM). However, at the start of the episode we see the full moon at night (approx. 19:50). Due to the fact that these are consecutive days, this situation is impossible from an astronomic point of view.
Factual error: The last number on the truck's license plate is "77." On Russian plates the last two/three digits stand for a registration place. "77" is a number for Moscow, but the Gulag is located in Krasnoyarsk region, so the number should be different. Besides, the format of the plate (LDDDDL|DD) is incorrect - the correct one is LDDDLL|DD (L - letter, D - digit).