You can challenge Roxanne an infinite amount of times (well, maybe not. I stopped after 4 times). Intended?
Edit 1: You can do that with pretty much all the Hoenn leaders.
Edit 2: Holy poo poo Zapdos!
Edit 3: I think legendary scripts are wonky. I saw Groudon on my first pass through the mountain. Also saw Suicune and Mew too. I have 6 badges and haven't set foot in Humilau City. Didn't see Articuno or Entei though...