For legal purposes (i.e. CE marking and EMC compliance) we have to design systems which have a minimum level of RF immunity. This level of immunity is adequate for most real world situations, but not all situations. I've been working on systems which often need to work right next to walkie-talkies, which means we are talking about more V/m than any test house can produce. We have to tolerate these fields because that's what the customers expect.

In terms of RF immunity, there is no worst case. I think you have to make it work up to acceptable levels, and above that you have to ensure it doesn't do anything unsafe.