The adjective fortuitous means happening by accident or chance. It’s synonymous with random and accidental. Fortuitous is often conflated with fortunate, meaning (1) bringing something good or unforeseen or (2) having good luck. Perhaps due to the words’ similarity in sound, fortuitous has long been used primarily to describe fortunate accidents rather than things that are merely accidental. Using the word to describe an unhappy accident would not be illogical, but it might confuse some readers.
The use of fortuitous as a synonym of fortunate without implying accident or chance, as seen in the examples below, is not in keeping with the word’s traditional meaning, but it is common:
Getting rid of your pain will merely be a fortuitous side effect. [Los Angeles Times]
Both are seen by conservatives as politically fortuitous events to be exploited rather than problems to be addressed. [The Nation]
These writers use fortuitous in its more traditional, dictionary-noted sense:
Dramatic overtime wins, fortuitous bounces, late heroics efforts have been the order of the day in past years. [Canada.com]
King spent 15 years living in Melbourne before returning to Britain, where another fortuitous meeting led to his big break. [Sydney Morning Herald]
Then a fortuitous event occurred: I ran into one of the focus group members at a party. [Mother Jones]
You might notice that each of the fortuitous events in these examples is positive. But remember, while we conventionally use fortuitous to describe positive things, positivity is not essential to the word’s meaning. It’s a connotation that modern writers and readers have added to the old word.