(a)
ser.ənˈdɪp.ɪ.təs: happened or found by chanceThe two scientists have made a serendipitous discovery.
There is a book on his serendipitous adventures.
arrive or increase volume
He turned up late again.
cause to happen
The protest brought about new reforms.
suddenly came to mind
The idea just popped into my head while I was shopping.