Wow, that sounds bad if it didn't correct itself after a minute or two. Mine normally returns to normal after a minute (which can seem an eternity if your holding up traffic). Also i have found that if it happens when facing downhill, I can let the car roll forward slightly and get it in second gear (first is not an option) then off we go (the snowflake still continues to flash for a minute but it changes up the gears normally). Although it doesn't happen every journey, if it's going to happen it seems to be after driving for 45 mins. It has occurred once or twice when starting from cold but very rare. Like I said, I have mentioned it to a garage who put it on a fault finding computer and nothing came to light. It wasn't a Citroen garage though, so maybe Citroen garages have more compatible equipment.