"With a large enough sample, any outrageous thing is likely to happen" (Diaconis and Mosteller 1989)

law of truly large numbers (coincidence) ... Skeptics Dictionary
...... Reader Correction

Law of Truly Large Numbers ... Wikipedia

Littlewood's Law of Miracles -The law of truly large numbers

The birthday problem is often used to show why we need not be so surprised when a coincidence does occur. The surprise is the low number of people involved. In a group of 366 people, ignoring leap years, it is certain that at least two people will share the same birthday. Yet it takes only 23 people for a 50% chance that at least two of them will share a birthday.
To see how this works click here

Some interesting articles on coincidence follow:

Coincidence
...... Interactive Mathematic Miscellany and Puzzles

Why People Believe in ESP for the Wrong Reasons
…... Sharon Presley

Coincidences: the truth is out there ... Robert Matthews

The Power of Coincidence … Robert Novella

Coincidences: Remarkable or Random … Bruce Martin

Unlikely Events and Coincidence
Unlikely Events and Coincidence / Page 2
...... International Cultic Studies Association (ICSA)

Numeracy