A random event or thing that, quite frankly, is creepin'. Lurking variables can occur at any time, and be used as an excuse for almost anything. Only really hairy math teachers can use this term to its full potential.
-"Billy, why were you out so late last night?"
-"Lurking variables, Mom."
-"Crikey! That crock almost bit me in my lurking variables!"
-"Now class, the lurking variables in measure four are being played wrong, remember the lurking variable rule. If a note has a lurking variable next to it, it will be played that way throughout the measure unless another lurking variable occurs within that same measure.