Failed Emmission Inspections in California

[DC1]

I'm about to buy my first Boxster, I have noticed in running various carfax's that a few of the vehicles I was interested in failed the CA emmissions, they all passed approx 50 miles later but I was wondering is this a common problem or a sign that the car has issues? I had focused on 5 different vehicles in my area and two of them had failed inspections at one time, I found that troubling but I wonder if it is common? Also and this should be another thread, assuming I have an inspection is 50,000 miles give or take dangerous for a 2001 vehicle? Thanks so much.