(ShareCast News) - Oil stocks led to the downside on Friday after the US Federal Reserve's rate decision on the previous day fanned global growth fears.Traders reacted poorly to Fed chair Janet Yellen's admittedly difficult to deliver message regarding growth concerns for overseas markets, on the one hand,while holding out the prospect of a hike in rates as soon as next month.Despite that, as of Friday traders in the market for Fed funds futures were placing a probability of just 52% on the first tightening move in this cycle arriving in January 2016, versus December 2015 before last night's announcement.While it might seem like US rate-setters are splitting hairs, the past financial crisis showed just how quickly cracks in financial markets can be indicative of more serious underlying strains in the system. Speaking on Bloomberg TV, Bill Gross, the portfolio manager for Janus's Global Unconstrained Bond Fund, highlighted financial strains as the key worry for the US Fed.Gross also pointed out that central banks the world over have precious little 'ammunition' left to fight against renewed downturns.Against that backdrop, the selling pressure piled up again on the likes of Glencore and Weir. The oil majors also endured sharp falls in their stock, with BP moving lower by over 3%. Royal Dutch Shell was also pummelled, with the shares off by almost 4% in the case of the A shares.The situation was much the same on the second-tier index, with large drops in Premier Oil, Kaz Minerals and Tullow Oil.Losses in the oil markets however were rather tame in comparison to the price-action seen over the last few weeks.Making matters worse, just after noon what appears to have been an error with an automatic trading system sent several London-listed blue chips into free-fall.That dragged the benchmark 60 points lower before automatic circuit breakers kicked in for BP, Vodafone Group, HSBC, Rio Tinto Group, BT Group, BHP Billiton, National Grid, Diageo and British American Tobacco.The London Stock Exchange said it would investigate the matter.