Incidents are bound to happen wherever a large group of people get together. But when you're talking about fans as those of Zagreb, I don't think you can call it "incidents".
Last time I was at an "away" game, there were 20.000 of us. Incidents did happen obviously, but there was no trace of violence.
(Note: I myself am not a fan of the team that was playing then, I was just tagging along with friends as I do every other week or so.)