Revealing mistake: When the taxi goes over the edge to capture Dr. Neville, you can see that the underbody of the car as it goes over is a flat painted surface and not the underside of what should be there (i.e., muffler, gas tank, etc.).
Revealing mistake: When Dr. Neville first approaches "Fred" at the trap, we see the taxi from both sides as it sits on top of an intact railing. When the taxi starts to teeter over, there is now a section missing in the bridge that was cut out for the car to fall properly off and be at the right angle to "teeter" as well.
Answer: I thought people jumped on another helicopter causing it to spin out of control, and that one crashed into their helicopter.