#Fail: 15 tech flops of 2015

Investors have been signaling warning signs that private companies are overvalued.

Upfront Ventures investors Mark Suster wrote in October, "Either we've discovered magical beans and elixir or perhaps we've gotten ahead of ourselves on valuation."

Square, Etsy, Box and Fitbit all went public this year but their stock prices have struggled. They may have been overvalued as private companies.

There are more than 140 privately-held companies valued at $1 billion or more. But many of these companies -- dubbed "unicorns" -- are unprofitable and focus on a "growth at all costs" mentality that may not weather market turmoil.