Well that is not surprising, now is it?
Someone I know closely recently planned to visit Paris. When they had checked in on the Eurostar at St. Pancras Station, and were at the immigration window, did they they realize there was a problem. The officer pointed out that they visa started 2 weeks later.
The officer then called a policeman, who escorted them out of the Eurostar area, and gave them a ‘refused entry’ paper. Not sure if this will affect their future visa applications.
But the lesson is, be sure that you don’t try to travel to the Schengen area before the visa start date.