However if Kerry wins by 270-268, then if only one of Kerry's electors abstains from voting, then the tally would be 269-268 and without the magic 270 number, the decision would go to the House again and Bush would win.
Another possibility is if Kerry wins by 270-268 and one of his electors is considered ineligible for some reason, then again the count would go to 269-268 and Bush would win.
This last possibility could happen. One of Kerry's Ohio electors, Rep. Sherrod Brown, is constitutionally ineligible to be an elector because he is a federal officeholder. He resigned yesterday as elector, thankfully. However, we would have to expect the Republicans to try to get Kerry electors declared ineligible if its a close election. As long as Kerry doesn't get 270 electors, he will lose.
A scary possibility suggested was the following: Suppose the EC is tied 269 to 269 and the House deadlocks 25 states to 25 states. This is exceedingly unlikely, but just suppose. Then the Senate gets to choose the vice president. Also suppose the new Senate is divided 50-50, a very real possibility. Then the sitting vice president, Dick Cheney, gets to cast the deciding vote, electing himself as the new vice president. In the absence of a president, Cheney would be acting president for four years. This is not likely to happen because the Republicans are virtually certain of controlling at least 26 state delegations in the House. Still, scenarios like this one support the case for electoral college reform.