Accidents are events (and their consequences) that are NOT intended to happen. They have not been in the Design or in the Grand Plan, but for one reason or another they happen.
A Perfect Design is quite unique. You are probably aware that for every good design there are many, sometimes infinitely many bad designs.
Hence, it is very important to design the Universe as a "control system" that is self-correcting, or preferably self-perfecting. For more details please read this article.