Revealing mistake: When the family's car is about to be hit by the ambulance as they try to escape downtown, Gerry is driving and his wife is up, leaning over the passenger seat talking to their daughter. The camera angle changes to a view from behind, and as the ambulance is about to hit, we can see there is nobody in the car and the ambulance driver is wearing a safety helmet. This is viewable in just a few frames. (00:08:00)
Revealing mistake: During the first major panic scene downtown, as the family get into the RV, one of the shots shows a man in a blue hoodie being 'changed' by the virus. He writhes on the ground in front of a taxi - the lettering on the door of the taxi is backwards, showing it's a reversed shot. (00:10:45)
Revealing mistake: When Gerry cuts the woman's hand off you can see it's an obvious fake arm and stump.
Chosen answer: The significance of showing the blood drop on Gerry's tongue and him spitting it out was to back up the theory that the only way to become a zombie is to be bitten by one. After the blood touched his tongue he ran to the edge of the building and waited 12 seconds (the time it takes to become a zombie) before he stepped off the ledge. If he would have started to change he was going to attempt to jump off the roof before causing harm to his family.
Then how does the scientist at the WHO get infected from a blood sample?
You can't turn by ingesting, or getting the blood on you. It has to enter your bloodstream. The scientist cut his hand and got zombie blood into his bloodstream.
He stuck himself with an infected needle or a "sharps" tool coated with infected fluids, which cut him.
Blood tester is encased in glass. Dr. Testing did not pay attention. His hand was in the case while it was mixing. Glass broke, cut him, testing blood mixed with his.