Shares in two of Europe's biggest companies veered more than 6 percent from their market price earlier today, swings that in one case affected levels in the benchmark gauge for European equities before it was reversed.

HSBC Holdings Plc jumped 9.9 percent to 688 pence around 11:20 a.m. in London before falling back to 629 pence a few minutes later. The increase was mirrored in the Stoxx Europe 600 Index, which momentarily pared its losses by 0.5 percent during the period. About two hours earlier, Diageo, the world's biggest distiller, dipped 11 percent to 1,691 before erasing the loss over a five-minute stretch.

"Looking at the speed of the correction in the shares, it's pretty clear that it was a fat-finger trade," Alastair McCaig, market analyst at IG in London, said. "There will be costs associated with that, so someone is in a fair amount of hot water."