As the only Order member to ever defect to Voldemort, Peter Pettigrew was automatically the weakest of all Order members. His defection was made worse by the fact that as a member of the original Order and a longtime friend, James and Lily Potter made him their Secret-Keeper for the Fidelius
Charm protecting them and their son Harry.
Pettigrew divulged their secret location to Voldemort, allowing the Dark Lord to find the Potters, eliminate James and Lily, and nearly take the life of baby Harry as well. The actions of Pettigrew also caused the Order to incorrectly believe for years that one of their most loyal members -- Sirius Black -- betrayed them and had secretly been Voldemort's man all along. Sirius spent twelve years of his life incarcerated in Azkaban and enduring the Dementors as a result of being framed by Pettigrew.