Abandoned City is one of the Briggs missions in which at the very last moment of the mission, when Briggs and Sam are sitting down guarding the capture target, the game glitches and you end up just sitting there indefinitely.
This has occurred both times I've attempted to complete Abandoned City on perfectionist mode, and was curious if they had patched/fixed this yet?
you don't have to restart the whole mission. you can just Reload at Checkpoint and it will be fixed...
I know this because I spent two hours using 2 controllers to finish that level and I glitched at the last part. I thought I'd have to redo everything but it went okay after I reloaded at the checkpoint.
Imagine I had to kill every enemy by myself and stop some of the drones more than twice. @_@