NYSE officials said in a statement that investors and customers should use NYSE-only closing prices for all NYSE-listed stocks for that trading day. All NYSE-only closing prices from Wednesday were correct, it said.

'What apparently happened was that they had some trouble with their trading of listed securities over Nasdaq and for whatever reason the volume of activity created technology challenges for them,' said Larry Tabb, an analyst at The Tabb Group in Westboro, Mass. 'For whatever reason, they had a technology problem.... That problem could have occurred by moving [new technology] into production that morning, or they had an emergency and moved [some new technology] into production during the day that shouldn't have been moved, or [maybe] an errant transaction just blew them out of the water.

'It means that for whatever reason they had a problem with the SIAC feed -- and that means that all of their trade reporting at that time basically stopped,' Tabb said.

When Nasdaq restarted everything late in the day, it resent the whole trading file instead of culling out all of the transactions broadcast Wednesday morning, he said. 'What they did was just resubmit everything, and because they just resubmit everything in original format SIAC didn't know that these transactions were duplicates. Because of that, [SIAC] picked them up as live transactions which then got distributed around the street, which then confused people even more,' Tabb said.

'So what Nasdaq is saying is that you should back all these errant transactions out of any historical databases, out of any historical files, any calculations of average price, all the last time and sale information. So Nasdaq is basically saying, 'We messed up and now it's your problem to try and fix.''