*air resistance
*inaccuracy of measuring instruments i.e. stopwatch i.e. its hard to tell when the pendulum is at its peak, and then press the stop button
there is also a parralax error with you viewing it cos your point of view on the pendulum will change if you move eyes, head body etc. The your reflex is also an error cos no one will ever get the correct results every time at the top of its motion cos of human reflexs.