When Rosita climbs on top of the ambulance, two variant walkers follow her, climbing onto the front of the vehicle. However, both are suddenly gone in the next shot after Rosita jumps to the drainpipe.
When Darly is donating his blood to Judith, Carol inserts the needle in Judith first, then into Darly without bleeding the tube of air. If this would of actually happened, it would have caused an embolism and could have caused Judith to have a heart attack, stroke, or respiratory failure. Without medical staff or equipment on hand, Judith surely would have died.
When Rosita falls from the pipe to land in the horde, a large black padded mat can be seen where she falls.
While "Judith" is getting pushed on the gurney to escape the Walkers, a doll is clearly used in place of the actress.
At approximately 33 minutes when they are at the gates, you can see some of the lighting equipment in the lower part of the screen.