Sol 4599 (!), December 31, 2016. Opportunity encountered soft ground on Sol 4591, and mission controllers rolled it back downhill about two feet while they figure how to advance without slippage.
In the meantime, the Pancam shot a multi-band survey of an exposure of bedrock lying on the left side of the rover (above; 5.9 MB). It also took a close, multi-filtered look at the wheel track where the right front wheel slipped in the soft ground (right).
Both images enlarge when clicked; both are false-color composites made by Holger Isenberg.
Opportunity raw images, its latest mission status, a location map, and atmospheric opacity, known as tau.