Channels around the sunroof frame carry the water away, leaves, dust and dirt block them, the rubber valve at the end of the channel sticks shut, and the water runs into the headlining, the A-pillar, the footwell and the boot. On the B8 A4 this often leads to electrical faults: you often miss the water and notice instead that something electrical starts behaving oddly, and by then the control unit in the floor is already corroding. Cleaning the drains is a cheap repair, replacing a soaked control unit is expensive.