If you walk into somebody in a place where you didn’t expect him, you say:
Hé, dat is toevallig!

What a coincidence!It also can be used ironically, when it isn’t really a surprise or if there is something fishy about it ….

Dat is wel heel toevallig

Well, well, what a coincidence…

We also use this word for ‘incidentally’ :
Weet iemand toevallig waar Jan is gebleven?

Does anybody incidentally know where Jan has gone?