((U+C+I) x (10-S))/20 x A x 1/(1-sin(F/10))
This article says the probability of Murphy's Law (What can go wrong, will) being exhibited can be calculated by the formula above. The variables are:
U = urgency
C = complexity
I = importance
S= skill
F = frequency
A = aggravation
Use a number between one-to-nine for all of the variables except "A" (which is set constant at 0.7). The interesting part is the assertion that you can control some of the variables and therefore affect the probability. YMMV.