With every launch of a new hardware you're going to see a lot of reports about defects and dead on arrival units, and PlayStation 4 Lead Architect Mark Cerny explained in an interview with Digital Trends the difference in philosophy between working on software and working on hardware, where most of the work involves avoiding risks as much as possible.

After mentioning that the software team will try to put as many features as possible into a game, and then if some of them won't work correctly, there's always the possibility to solve the problems with a patch, he went on to explain that working on the hardware team is completely different:

But with the hardware team it’s all about risk. You get one shot at it, and if it doesn’t work it’s potentially a six-month schedule to go fix it. So the hardware teams think about risk. Anything they talk about is “what is the risk of doing that?” Whether it be a feature set inside of the chip or the power consumption of the chip, or the like.

He even went as far as mentioning the Xbox 360's red ring of death, explaining that a large part of the mission of the hardware engineers working on a console is to avoid that kind of problem:

Just to give one story – and this is not about us – but a competitive hardware had a red ring of death… My belief is that it’s the custom chip, because it is so hot that it expands, and when it cools it contracts and it actually walks back the pins – the pins walk off the pads they are connected to after a certain number of heat cycles. And that was a $2 billion public cost to address it. And so much of what you are doing with hardware engineering is trying to avoid the risk that anything like that may happen.

For now Sony reported a 0.4% of faulty units between the initial batch of shipped PS4. We'll have to see if that percentage will remain constant (which is very unlikely if not impossible, as it'd mark the console as one of the most flawless pieces of consumer electronics ever built, which isn't something I'd expect from something assembled by interns at Foxconn) or will raise over the short, medium and long term, and by how much it'll raise.