accidently pushing all in

Sounds a bit mad this one but it did happen. 2 players left after the river and a bet was placed and called but the chips were not pulled into the pot but left in front of the players. When the winning hand was shown and player A had won player B pushed his final bet into the pot and player A took it. Then when player B sat back he noticed that he had pushed over half of his remaining chips into the pot by mistake. I was asked to rule on it and my ruling was that he put his chips across the playing line so they were in the pot. Any advice on other ways of dealing with this situation.