4:30 PM PT -- Olivia is leaping to Chance's defense and says he was merely pulling the ole "scratch me on my leg" move.

"Chance, nooooooo!!!"

... Is what Olivia Munn probably should have shouted as her rescue dog greeted a greeter at LAX by marking his territory and peeing on her left leg.

Munn was flying out of the airport Wednesday for Thanksgiving with Chance and her other dog, Frankie. Clearly, one pooch is more of a bad boy (but they're both good boys).

Olivia didn't seem to notice and the LAX employees didn't seem to mind ... so airport crisis averted.

The actress and her pups strolled away afterward, all smiles.