The phrase "just a coincidence" is commonly used to dismiss an odd occurrence as something unimportant... but actual reported coincidences may not be unimportant. A coincidence is generally when two unrelated events seemingly have a connection that they shouldn't. Sometimes, it is the timing of the events that seem to connect them, such as saying "I'm hungry," then having a coupon for a free sandwich blow up against your leg. Sometimes it is the context, such as speaking of a person you rarely see shortly before they call. On rare occasions, people will report an urge to go to a place they wouldn't normally go, where they find something they needed and would not have found otherwise; in these cases, the connection between the events starts to look decidely strange... is the person being led to what they need, or did they somehow know where it was before they went? Mark Twain himself studied events like these, convinced they might be proof of unknown powers of the human mind.
Coincidences shouldn't be mistaken for prophecies or omens, although all three categories sometimes blend together. In a prophecy an event is actually predicted to happen, where in a coincidence there is no such attempt at foreknowledge. Omens, events believed to signal ill-luck, also imply foreknowledge in the form of a warning, which also does not occur with the simpler coincidences.