Kate Walsh Melts Down at LAX Over Bag She Left Behind on Flight

Kate Walsh damn near had a meltdown when she realized she left behind a bag on her flight ... though you couldn't tell by TSA's sloth-y reaction to her anxiousness.

We saw Kate shortly after she landed Thursday at LAX, but before our guy could ask a question, she went into sheer panic. The issue? She forgot one of her bags on her Delta flight, and desperately wanted to go back to get it.

Check out the vid ... she was clearly NOT having it with Delta, LAX and TSA staffers who wouldn't let her back into the gate area after exiting. We all know that's absolutely forbidden once you walk out.

Kate, however, eventually talks her way back in ... at least to the TSA checkpoint -- and you gotta see the agent's reaction as she continues her rant. Kinda hilarious.