When is it permissible for the congregant to intend to separate from his imam while the validity of his prayer is preserved?

The general rule is that the ma'mum (follower) must follow his imam (leader) throughout the entire prayer, as the Prophet (peace be upon him) said: "Indeed, the imam is appointed to be followed." However, there are cases that permit the ma'mum to separate from his imam before the taslim (salutation of peace) and complete his prayer alone.