When I think of "coincidence" in Romeo and Juliet, I also think of misunderstandings or miscommunication. There are so many times in the play that an impulsive act results in death because of lack of communication. In that case, that could mean that the characters had more control over their actions than thought. The problem with the word "coincidence" might be that it should really be substituted with "Fate". When does Fate intervene and when does it not? The Prologue is clear when it says that these two lovers will not find a happy ending, so no matter what they do to stay together, by pre-destined fact, will not help them. So, I'd say first, Fate messes with them the most, thereby creating the illusion of coincidence.
One could argue that if Rosaline had not rejected Romeo, he wouldn't have been sad so he wouldn't have gone to the party to meet other girls. Then, it could be said that Romeo's choosing to fall in love with Juliet was a coincidence because there were other girls there at the party. Next, had Tybalt not wanted revenge, then the fight wouldn't have happened. One last major coincidence was when Friar John wasn't able to get the message to Romeo about Juliet faking her own death. He was held up by a quarentine in the house that was beyond his control. Had he been able to get the message to Romeo, then the lovers wouldn't have died.
Look at your assignment and narrow it down as needed. That is a big question.