The lost to City away from home. Wouldn't say they 'blew' their chance of opening a 10 point lead.
City weren't on great form though at that time, just lost to Palace, lost to Leicester, they were wobbling then. They weren't great in that game against Liverpool either, just more clinical. Lost to Newcastle shortly after.
December/January were difficult months for City, and Liverpool didn't capitalise. Even a draw would've been good and would've left them in a commanding position.
Liverpool have a bit of a history of doing this in the PL. Didn't they have a chance to go 6 points ahead of us in 08/09 around January sometime and instead drew to Stoke 0-0? Around the same time Rafa went on that presser rant?
Not to mention they're epic choke against Chelsea in 14' when they only needed a draw in that game. Chelsea had a makeshift side as well hamstrung between two CL semi finals against Atletico.