When a writer is already stretching the bounds of reality by writing within a science fiction or fantasy setting, that writer must realize that excessive coincidence makes the fictional reality the writer is creating less 'real.'
Diana Wynne Jones' excellent book 'The Tough Guide to Fantasyland' is a compendium of the sort of lazy writing that has given fantasy fiction - especially the sub-section that features elves and dwarves and other Tolkienesque elements - a bad name.
The sad thing is that, for many writers of fantasy fiction, the inclusion of magic seems to mean that logical ramifications and real-world laws both go out the window.