Plot hole: Agent Spender watches in horror as Alex Krycek stabs the neck of an alien directly in front of him and the usual bubbling green blood spills out. In every other instance of a human being in such close proximity to the green blood, the person has immediately taken a painful reaction to the substance, including severe damage to the eyes. Spender however is completely unaffected, as is Krycek, who was also close enough to feel the effects.
Continuity mistake: When doctors make a cut on Cassandra's belly, the green substance comes out of the cut and pours down directly onto the piece of blue cloth she is covered with. In the next shot the fragment of cloth that was in contact with the substance is dry, but in the following shot, there is a large soaked stain on the piece of cloth.
Plot hole: It is virtually impossible that such a conspiracy-experienced, cautious, and wise person as the Second Elder would be so naive and careless to just let the "Dr. Openshaw" faceless alien to his house, knowing from the CSM that Dr. Openshaw had died that evening.