Then we have the problem of on-site modifications.

...which overload the capacity of your system, causing it not to recognize that the gas levels have siginificantly increased in the last N minutes becuase it was sending out email to the PHB that wanted the change made to the control system in the first place.

Of course, this knee-jerk reaction is based only on limited reading, and not real world experience in the subject. I am not sure that I would have the needed level of confidence in the tools or my skill to be on that project :)