One major cause of failure for garage doors using extension springs is that the pulleys wear out. Usually the ball bearings break down, causing the pulley to scrape against the track angle, or the rivets fail and the pulley splits as in the picture above, causing the door cable to stick. 
When the owner of this garage door called us she said, "The garage door opens part way and stops." This is one of many symptoms caused by worn pulleys. This particular door would not sit evenly on the garage floor. When we tried to open it the garage door jammed. 
Generally, when pulleys fail, the garage door does not open and close properly. The garage door closes part way and reopens. The door binds and sticks, and scraping sounds often accompany the other symptoms. Some times the first symptom is a frayed or broken cable, or a dangling extension spring, or a dent in the car. In my 30 years servicing garage doors I have found these symptoms to be some of the more challenging for do-it-yourselfers to diagnose. 

 Since all the pulleys in the extension spring system are used approximiately the same amount each time the garage door opens and closes, we recommend that you replace all four pulleys - this will save you time and money if the future.