It is likely this won't affect many, but we have good evidence of counterfeit high voltage transistors from some suppliers. The types so far affected are BU505 and MJE8502, these are 1500V VCES and 700V VCEO. We noted failures in our products and traced to a very low VCEO which I checked with a current limited voltage source. We found ST brand BU505's failing at slightly more than 500V VCEO and the same for Onsemi branded MJE8502. I etched a couple of BU505's with nitric acid to remove all metal. One was a good tested one and one the other a failed one. There was a completely different die size between the two. The good one had a 3.3mm die and the bad one a 1.8mm die. The good one had the markings etched and the bad one had markings printed. These devices are often used for switchmode supplies and horizontal drive in CRT's, where circuit designs may not ever allow the base to be open circuit, ie driven from a low impedance source, so I guess that many users many not see the failures.
I presented the data to St Microelectronics and they confirmed the likelihood of counterfeit devices. Now we will have to do incoming VCEO checks of all batches until the situation improves. We try to get these normally from mainstream suppliers, but sometimes supply problems prevent this. Looks like we will have to get smarter with purchasing though. This is the first time I have personally come across this in 20+ years of engineering.