Hadn't heard of this problem with that particular aircraft but autopilot problems are not uncommon. There are hundreds of computers in a 777, many of them triple redundant. As with any computer, software errors and problems do occur. Luckily, the computers are usually each located inside their own black box that slides into a rack with one big connector on the back, called an LRU (line-replaceable unit). So when problems like this occur one of the first things the mechanics do (after rebooting) is to swap the problem computer out with a fresh one and send it back to the bench to be thoroughly tested. In all likelihood the computer that caused the previous error is/was no longer on the aircraft.