prov. The generalized or `folk' version of Murphy's Law, fully named "Finagle's Law of Dynamic Negatives" and usually rendered "Anything that can go wrong, will".
The label `Finagle's Law' was popularized by science fiction author Larry Niven in several stories depicting a frontier culture of asteroid miners; this `Belter' culture professed a religion and/or running joke involving the worship of the dread god Finagle and his mad prophet Murphy. Some technical and scientific cultures know it under the name Sod's Law; this usage may be more common in Great Britain.
Anything that can go wrong, will.