The are some "classic" signs of market bubbles in certain asset classes, according to VTB Capital's Global Macro Strategist Neil MacKinnon, who believes that easy money from central banks is to blame. "I think what we're seeing across some asset classes is the appearance of bubbles — we're thinking of cryptocurrencies," MacKinnon told CNBC Tuesday, also mentioning the market for fine art. "There are bubbles and I think they arise from ultra-easy monetary policy that has created a speculative excess and excess leverage." MacKinnon is not alone in this view — many market commentators have warned of the downsides of enduring loose monetary policy, calling for an end to quantitative easing in the face of resurgent global growth. "There is growing concern among some market commentators, even among central banks, that there is a potential threat to financial stability," he went on. "I think the (Federal Reserve), for example — they are concerned about the risk to financial stability, and they should be." The Fed has forecast three rate hikes in 2018 while targeting a range of 1.25 percent to 1.5 percent for its benchmark interest rate, inching up from years of historically low rates as economic growth strengthens.

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"It could well be that we see more frequent flash crashes, that's for sure. Worst case scenario is an outright market crash — it may come from underneath the radar screen as sometimes they do," MacKinnon said. He ruled out risks in the banking sector, however, noting the sector is well-capitalized and banks have raised their liquidity buffers.

'Stability brings about instability'

Instead, he suggested, the threat may come from historically low volatility. "Many commentators have pointed out that the 'big short' this time around is in Vix volatility," he said, referring to the CBOE's headline index measuring volatility in U.S. equities. The Vix has been significantly below its historic average of 20 for most of 2017. Paradoxically, the apparent comfort level in the markets may actually be a recipe for impending shocks. "The super low levels of volatility are very vulnerable either to an inflation shock which then brings about an unexpected interest rate shock," MacKinnon stressed. "These low-volatility strategies have created fragility — stability brings about instability." Others in the market sphere might not have such strong views, though they emphasize that investors shouldn't expect the coming year to be a repetition of 2017. Deutsche Bank expects the global recovery to continue in 2018, projecting global growth at 3.8 percent compared to 3.7 percent in 2017. However it is warning investors not to assume economic growth will automatically translate into similarly high levels of investment returns in the year ahead. "Forewarned is forearmed" was the first of 10 themes in the bank's new report this week.