Synonyms happen (to take place): Accidents like this happen all the time.
occur formal to happen, especially unexpectedly: These problems occur in all families.
arise (to begin to occur or to exist): We will contact you when the need arise for extra staff.
come up to occur or present itself: Opportunities like this don’t come up every day.
come about (to happen): How did the problem come about in the first place?
crop up (to appear or happen unexpectedly): In our office, new problems crop up every day.
materialize (to take place): The bad weather condition predicted, never materialized afterwards.