Look at how GoPro hit the wall on revenue growth

Wednesday was a terrible day for GoPro, the action-camera company. It warned that sales for the last quarter of 2015 would be lower than expected and said it was laying off 7% of its staff.

The stock was halted, then plunged as much as 20%. It's now down about 14% from before the news. Overall, it's down more than 80% from its peak, in October 2014.

One big problem: GoPro's growth has been slowing on a percentage basis every year since the company started. Growth often slows as companies earn more revenue - it's harder to double from $1 billion by finding another billion in sales than it is to double from $100 million (that's the law of large numbers) - but that slowing growth couldn't justify its high price-to-earnings ratio, which peaked at over 80.

Now it's down around 10, which is typical of very large tech companies like Apple.

As investor Marc Andreessen said, hardware companies have to protect themselves against commoditization; otherwise they'll end up like Flip - the last great mobile video camera company acquired by Cisco in 2009 for $590 million but then unceremoniously shut down a couple of years later, after everybody started taking perfectly fine videos on their phones.