http://avherald.com/h?article=41adf38c

An Air Canada Boeing 777-200LR, registration C-FIVK performing flight AC-856 from Toronto Pearson,ON (Canada) to London Heathrow,EN (UK), was enroute overhead the Atlantic near N50 W46 at FL390, when the crew reported problems with the autopilot without declaring emergency and requested to return to Montreal,QC (Canada). The airplane descended to FL340, turned around and headed back to Canada, when the crew requested to return back to Toronto. The airplane landed safely on Toronto's runway 15L about 4 hours after turning around.

The captain told the passengers, that the navigation computers had fallen into degraded mode.