Would it be reasonable to say that the following scenarios are more probable?
* The specific version of the software that was used to generate the non-prime contained a bug in its implementation of the algorithm
* The specific hardware upon which the software was run was flawed in a way that generated an incorrect output number from the program (i.e. on-disk corruption, RAM corruption, etc., undetected by the checksums in the hardware)
Both of those scenarios are more probable. But neither is anywhere nearly as probable as human error or human malice.
In the case of human error, it could be as simple as a bad cut-and-paste. In that case the number likely has a half-dozen or so factors, some of which should be found relatively easily.
* The specific version of the software that was used to generate the non-prime contained a bug in its implementation of the algorithm
* The specific hardware upon which the software was run was flawed in a way that generated an incorrect output number from the program (i.e. on-disk corruption, RAM corruption, etc., undetected by the checksums in the hardware)