Ford was not going to be outcooled. "Zaphod," he drawled, "great to see you, you're looking well, the extra arm suits you. Nice ship you've stolen."
Arthur goggled at him.
"You mean you know this guy?" he said, waving a wild finger at Zaphod.
"Know him!" exclaimed Ford, "he's ..." he paused, and decided to do the introductions the other way round.
"Oh, Zaphod, this is a friend of mine, Arthur Dent," he said, "I saved him when his planet blew up."
"Oh sure," said Zaphod, "hi Arthur, glad you could make it." His right-hand head looked round casually, said "hi" and went back to having his teeth picked.
Ford carried on. "And Arthur," he said, "this is my semi-cousin Zaphod Beeb ..."
"We've met," said Arthur sharply.
When you're cruising down the road in the fast lane and you lazily sail past a few hard driving cars and are feeling pretty pleased with yourself and then accidentally change down from fourth to first instead of third thus making your engine leap out of your bonnet in a rather ugly mess, it tends to throw you off your stride in much the same way that this remark threw Ford Prefect off his.
"Err ... what?"
"I said we've met."
Zaphod gave an awkward start of surprise and jabbed a gum sharply.