Coincidences happen every day in real life. In fact, things happen against all odds, every second of every day. So why shouldn’t you have a coincidence happen in your book? You can, if you do it right, if it’s meaningful. But if you’re not carefully laying foundations for yourthen your characters end up as nothing but puppets dragged around by strings.
The beginning of a scene should lay out the characters, their relationship, the environment and the basic conflict. A common mistake is to feel the need to establish all the information at the top of the scene. Let the exposition happen naturally. Let the readers discover who and where everybody is and what their relationships are. That’s half the fun.