First off, shares have not yet changed hands. The two parties to the trade each have an agreement with clearing houses that the shares will change hands in the next 3 days, modulo various clauses.

One of the clauses lets the exchange break trades under certain circumstances, typically in the case of obvious errors and "unusual circumstances". So it's fairly likely that trades above and below a certain price on some stocks will be broken.

See Rule 11890 (c) (2) of the NASDAQ.

http://nasdaq.cchwallstreet.com/nasdaqtools/platformviewer.a...

[edit: I previously attributed the NASDAQ rule to NYSE.]