Spankees losing 4 straight to Bos, Bos blowing a 14 game lead in August in 1978, but the 1964 Phillies is probably bigger than this Mets collapse:
However, from a 6½-game lead on the Cincinnati Reds with 12 games remaining in the season, Philadelphia collapsed in a 10-game losing streak (the first seven played at home). The crucial series came when the now second-place Phillies traveled to St. Louis to play the Cardinals after their losing home stand. They dropped the first game of the series to Bob Gibson by a 5-1 score, their eighth loss in a row, dropping them to third place. The Cardinals would sweep the three-game set and assume first place for good.