Controversial & Contradictive
Sound like the ECU was having difficulty calculating the air/fuel ratio to use. The 3500 rev at standstill was unexpected, and it was in the process of adjusting the fuel mixture to be rich due to the engine being cold. When it got the high revs, it had to recalculate the fuel mixture and probably just experienced a moment or two's indecision, which caused the rough idle. It's not that difficult to confuse the ECU for a second or 2 just by doing something very unexpected, so I'd not worry about it too much. If it happens regularly during "normal" driving, then start to worry.
53 A4 2.5TDi Quattro Sport 180 Tiptronic. Heated Buffalino leather, DVD Sat Nav RNS-E, TV, Bluetooth, 6 cd, BOSE, Colour DIS, Cruise Control, Auto lights and mirrors, Interior light pack, Heated mirrors.