According to
this MSNBC article
The most common type of reported problem, according to non-partisan observers, occurred when voters turned up at polling places and found there was no record of their registration.
The problem likely stemmed from the unprecedented wave of voter registration activity in the weeks and months leading up to the election. Under a 2002 federal law, voters who encountered registration problems were entitled to "provisional" ballots, which will be set aside and may or may not be counted. In Pennsylvania, some polling places reportedly ran out of provisional ballots and had to be restocked.
Will this be our Achilles' heel? Even if Kerry wins Pennsylvania and Ohio, will the GOP attack these "provisional" votes and have them thrown out? Will they even get counted at all?
I trust them (the GOP) about as far as I can throw them.