If anything can go wrong, it will.

(Other Authors and Sources)

“Murphy’s Law” (1949)

Direct variants:

• "Anything that can go wrong, will go wrong." 
• "Everything that can possibly go wrong will go wrong."

The history behind Murphy's Law -- and its very similar antecedents -- is long and disputed, unsurprising given its simple sentiments. It is most often attributed (via the name) to Capt. Edward Murphy, a development engineer working on rapid deceleration g-force tests, and first named as such by Dr. John Stapp, a US Air Force colonel and Flight Surgeon overseeing the project. See Wikipedia for a summary, and AIR for more information.