* Window regulator clips.
* Mass airflow sensor.
* Brake lamp switch.
* Hazard switch / turn signal flasher.
* Door latch / open-closed detector.
* Brake pads that wear out very quickly and leave lots of dust.
* Coolant temperature sensor.
* Stereo with volume control problems.
* Coil packs (1.8T engine).
* Short life timing belt (TDI engine).
So a surviving a4 car may have the newer more reliable versions of these common failure items, because the newer versions were available when the original parts failed. On the other hand, they could still be subject to other issues:
* Peeling soft touch interiors.
* Oil burning (2.0L engine).
* 4-speed automatic transmission unreliability.
* Some maintenance items are a lot of work (TDI timing belt).
* Glow plugs and glow plug harness (TDI engine).