As investors cheered the S&P 500 index forging another record last week, market pundits are warning of cracks starting to emerge in the world’s most watched sharemarket.

The blistering rally, which has pushed the US benchmark up more than 30 per cent from its October low, has faced little resistance, with just one hefty pullback during that period – a 5.4 per cent drop in April – while the other 12 market sell-offs have been less than 2 per cent.