randomIt was a random encounter between two long-lost friends.
chanceA chance meeting brought them back together.
arbitraryIt seems like the decision to cancel such a successful TV programme was completely arbitrary.
coincidentalAny similarity between the characters in this book and real people, living or dead, is purely coincidental.
inadvertentA good editor will catch and correct all inadvertent typos in a text.
unintentionalThe omission was completely unintentional, I can assure you.
accidentalHe was charged with accidental discharge of a firearm.
fortuitousThrough a series of extremely fortuitous events, she landed a job, an apartment, and a new car all in one weekend.