“Coincidences are a normal part of the natural series of events [waking, eating, shopping], but as a rule they should not be made part of the dramatic series [events that connect to form a meaningful story].” -- Damon Knight in Creating Short Fiction.I wouldn't be surprised to learn that most beginning writers fall into this trap, mostly with endings. I've read stories submitted for critique where the main character is saved from financial ruin by an uncle who unexpectedly dies and leaves her a million dollars, or where the villain is struck by lightning just as he's about to slay the hero. These endings are unsatisfying. The same is true for coincidences in the beginnings and middles of stories.
Coincidences sneak into my early drafts. It's an easy way out of a scene where I'm stuck. If I realize what I'm doing (and I usually do), I place a note in the text to remind myself to fix the scene later. I don't want to get bogged down in a first draft figuring a way out of an unexpected event.