it will vary from bookie to bookie but the vast majority will state in the terms and conditions they can cancel bets that include line errors at any time, upon signing up to them. it must happen now and again with bigger bookies bearing in minds the thousands of markets they offer each day. maybe people on betfair forum
http://community.betfair.com/ or sportsbook review
http://www.sportsbookreview.com/forum/ will remember specific events it's happened on. also I heard of a few times that they mixed up the names of the selections in a market and had to void all bets(might of been on an exchange tho), and I remember a ufc card a while ago where both nog brothers were fighting and some bookies had the names the wrong way round as they are near identical haha, I think the bets got voided then and lines reset, someone else may remember more than me on that though.