I was watching Doctor Who for the millionth time and I realized something
the eleventh doctor always says "never ignore a coincidence unless you're busy"
and that could have come from what happened to Donna.
around her was an acquaintance who took her to the Tardis, to the doctor and to all the events that would lead her to the episode "Journey's End".
the doctor didn't see the coincidences and when I went he didn't care. He only realized what was happening at the end (actually the clone that came from his hand noticed it first)
So it must have been after these events that he started using that phrase.
it was probably just "never ignore a coincidence", but since the doctor is the doctor he had to have this condition.