Sod's Law

Often spoken by project managers to express that no matter what they have planned for, Sod's Law will most probably still strike their project with something of relative certainly not panning out as expected. 

E.g. “We were all on track to do the big cutover on the night, when the network admins got trapped in the elevator - then fell ill from some dodgy take away they treated themselves before coming in – that’s Sod's Law”