State laws (often) make having one county or city inside another illegal - though they do it. They problem is that if you overlap authority then politicians can use a finger pointing game - and certainly will. Another thing they overlap that they shouldn't is various administration jurisdictions - which again I would say state law says not to do (school district, aid district, voting district, other). I had an experience once when 3 agencies said it was in the other agencies district (circle jerk) and they were "not allowed to talk to me".

It's illegal for a reason.