Fan in Fan out problem
Fan out is a very essential factor because when the load exceeds the fan out the gate will not be able to drive the load at the designated current. This excerpt from wiki explains this better,

An ideal logic gate would have infinite input impedance and zero output impedance, allowing a gate output to drive any number of gate inputs. However, since real-world fabrication technologies exhibit less than ideal characteristics, a limit will be reached where a gate output cannot drive any more current into subsequent gate inputs - attempting to do so causes the voltage to fall below the level defined for the logic level on that wire, causing errors.
The fan-out is simply the number of inputs that can be connected to an output before the current required by the inputs exceeds the current that can be delivered by the output while still maintaining correct logic levels.

http://en.wikipedia.org/wiki/Fan-out#Theory