When I took art in college, I learned a valuable lesson-- never point out "mistakes." Professors tended to always reward choices over admitted mistakes and excuses. Design like life, is about working with whatever life throws at you-- expected or unexpected. Do not fight for control over something you have absolutely no control of but rather learn to create design that works with it. You will find that much like professors, the universe will reward you.
The "happy accident” can include anything from your accidentally spilling paint on a canvas that you carefully and methodically prepped, to your basement flooding, or to a store not carrying the paint color you had in mind. Happy accidents should always be embraced and turned into “artistic" choices and new directions. You just might be surprised at how relinquishing a bit of the "presumed" control you have and just enjoying the creative process works to your favor.