A co-worker of mine just had an expensive experience with her Volkswagen. 

The car has the two liter twin cam engine, with a timing belt. The car had 85,000 miles on it, and had the original belt. This engine has five valves per cylinder, and it is an interference engine, meaning the valves will hit the pistons if the camshafts turn out of sync with the crankshaft. 

Unfortunately, the timing belt broke. When the belt broke, the valves hit the pistons, bending the valves. What should have been a several hundred dollar preventative maintenance repair, just became minimally a $1,200 repair. Minimally, the head needs to be rebuilt with new valves, if it can be saved. The head may well have to be replaced. The pistons could be damaged as well. 

If your car has a timing belt, it is not worth delaying the timing belt replacement, especially if it has an interference engine. A proper repair may involve replacing the water pump, camshaft seals and the front crankshaft seal.