The 1.8T, 2.7T and 2.8 engines available in the US spec A4/S4 usually require a timing belt replacement at 104,000 or 120,000 miles (check your owners manual for specific details about your engine). This is a critical member of the engine which should not be overlooked when its time comes due. These engines are what is known as "interference engines" meaning that if the belt fails you'll have expensive, fast moving bits smacking into other expensive, fast moving bits... namely, the pistons and valves. When pistons and valves contact they make some not-so-beautiful music together and cause substantial engine damage. Interference engines are quite common--this is not something specific to Audi.
Pictured below are and intake and exhaust valves from a 1.8T with a timing belt failure. I don't have any additional info about the incident but the pistons also sustained heavy damaged. If you don't know... the shaft of the valve should be straight. : o