In 2011, the Braves held a 10.5 game lead on the Cardinals on August 25th with a 79-53 record, projected to finish with 97 wins (would have been 2nd best record in NL). They even held a 8.5 game lead on September 6th. They proceeded to go 9-18 in September and finished the season on a 5 game losing streak. The Braves finished 89-73 while the Cardinals finished 90-72 to claim the NL Wildcard.
The Red Sox, in the same season, held a 9 game lead over the Rays on September 3rd with a 84-56, projected to finish with 97 wins (would have been tied with Yankees for best record in NL). They proceeded to go 6-16 in their remaining games. The Red Sox finished 90-72 while the Rays finished 91-71 to claim the AL Wildcard.
So both these teams went from World Series contenders to completely out of the playoffs in the last month of the season…