An unsubstantiated story from B742 days: a/c inbound UK from east after a long night flight from further east. No company routes, each WPT' inserted manually. Somewhere over the teutonic continent the a/c started to make a substantial turn around. Captain woke up and noticed the sun in the wrong place. Alerted F/O to the apparent anomaly. The 1st reaction was, "I know what I must have done, E instead of W for the next WPT or added a 1 to the E longitude." F/O dived into the 'box' to correct the error. Captain said, calmly, as he disconnected the A/P and pointed the a/c in roughly the correct direction, "let's get going towards home, and then you can mess about with the box of tricks." KISS.