I had bought 7 actually from this famous 11196-19-20G serie, SN A13510xxxxxxxx

Thay ran fine 24/7 for a few days... then one after the other 6 out of 7 were out.  Three of them in the spectacular purple smoke fashion described above. 

In the begining I was suspecting power variations and I ran out and bought a power stabiliser.  But that did not prevent another 7950 to burn right before my eyes.

Power supplies (by the way two of them do not work any longer following the purple somke episodes) were 80 plus gold or platinum. I did not overclock my cards while mining and tried to keep them as cool as possible with latteral fans blowing on both sides.  Temperatures stayed always below 68° C.

I checked out in the internet and apparently there are several similar stories with these cards. 

I had also several 270x from Sapphire in the same rigs that the faulty 7950 and they seem to run fine despite the sparks and smoke that burnt the 7950.

I also have other cards and not a single one of my other cards have had a glitch. I am not an expert but clearly the 7950 (or at least the 11196-19-20G serie) are unreliable compared to other models.