When Scully spots the bug in the dead prisoner's wound, she gets a pair of tweezers out of a pocket to pick it up. When the camera pans back to Scully's face as she examines the bug, she's wearing a face mask that was round her neck moments earlier.
At the beginning, multiple Turkey Vultures are shown squawking when they are shooed away from the carrion on which they're feeding. However, Turkey Vultures do not have voice boxes and cannot vocalize any song, call, or squawk, being limited to some crude hissing and grunting.
A Canadian speed limit sign "MAXIMUM 80 km/h" is seen during the gas station scene (16m:09s) but with the "k" painted or taped over.
It's stated that fourteen inmates of the prison are infected with the plague, but it's never explained how that could have happened. Only one package was sent to the prison, containing the boar's leg that infected the first patient. No one else received a package, so given that the contagion is spread by a pustule exploding onto someone else in close proximity, all of the sick prisoners would have had to deliberately place themselves in close contact with an obviously already-infected person, which is extremely unlikely. Moreover, the two escaped prisoners who eventually succumb to the disease don't seem to have any idea at all how they were infected.
When the doctors are examining the first dying prisoner, right after the opening credits, one reports to the other that the blood oxygen saturation is 81%. But the machine shown says it is 98%; the 81 on the machine is the pulse rate.
At one point Dr. Osborne tells Scully: "We finance exploration of new species for the rain forest with potential drug applications." As spoken this line doesn't make sense - it should be: "We finance exploration of the rain forest for new species with potential drug applications." It's unclear whether the error is the actor's or the writer's.
The medics wearing the full isolation suits and green gloves measure the boils and say that they are uniformly 5cm in size. Using his thumbs as reference it can be seen that the calipers are open about two thumb widths - less than 4cm.