 CONT: The design of the Enterprise model is different in some  exterior shots of the ship. Noticeable differences are that the  engine nacelles have a differently designed rear and front. In some  shots the front of the nacelle does not have the spinning light, it  is just a brown circle. Also in the rear shots the back of the  nacelle has many little holes whereas in other shots it is solid.  The reason for this is some of the shots are stock footage used from  the second Star Trek pilot "Where No Man has Gone Before". Since  then the design of the ship had changed, however the stock shots  were used many times in the actual series, hence the mistakes.