Artist's impression of a 1000km-diameter planetoid hitting a young Earth. Wikimedia Commons For as long as we've had a stock market, we've had the risk that it would crash.

In recent weeks, however, stocks have continued to roar to new all-time highs, far outpacing expectations for earnings growth.

As such, valuations have become frothy, and many signals are telling us that the market is being dominated by complacency and euphoria.

Because we may be in a bubble, the bears warn that we could see the stock market actually accelerate before we experience a violent crash.

"I still see end Q4 2013, through to end Q1 2014, as the window in which we see a significant risk-on top before giving way, over the last three quarters of 2014 and through 2015, to what could be a 25% to 50% sell-off in global stock markets," warned Nomura's Bob Janjuah in a recent note to clients.

What follows is a compilation of the warning signs that have been telling experts that the market is irrationally high and set up for a crash.