fortuitous / fortunate
By chance vs. lucky.
Fortuitous means happening by chance: a fortuitous discovery in the archive. A common error is to use fortuitous to mean lucky, as in a fortuitous win. For a lucky outcome, use fortunate: a fortunate win. A fortuitous event may be lucky or not.
Examples
UseOur fortuitous meeting at the station led to a contract. (by chance)
UseThe crew made a fortunate escape from the fire. (lucky)
AvoidThe crew made a fortuitous escape from the fire. (if you mean only that the escape was lucky)
Dictionaries now also list fortuitous meaning “lucky.” However, many people still consider this use an error, and they judge the speaker who uses fortuitous this way.