Definition of Jack in the Box:
Jack in the Box is a variable or an “x” that appears at random intervals during a process due to non-apparent external factors. Although this will not be focussed upon while creating a FMEA, the uniqueness of this variable is its ability to be significant enough to affect the process capability when it appears.
Example: During the benchmarking of a credit card process, the Jack in the Box was that employees of the bank randomly decided to apply for pre-approved cards for their family members. The number of cards issued were significantly large enough to affect the baselining. The cause could not be tracked because the applications happened across the entire bank for no apparent reason.