Random Left 4 Dead AI Glitch

I was playing Left 4 Dead last night on PC, trying to get the Untouchables achievement with 3 bots. On about the third or fourth attempt (the AI are SO bad at dodging the rocks thrown by the Tank), I was setting up the gas cans on the Runway Finale of Dead Air, when I noticed the AI were all in yellow health and fading fast. I hadn't even started the radio yet! So, I found them near the safehouse, bunched up and surrounded by zombies, just taking a beating. They wouldn't shoot or heal or anything. Even their dialogue seemed to be broken.

Now, I've been playing Left 4 Dead for the better part of five years. I originally had the games on Xbox 360, but re-bought them on PC when the 360's disc drive broke. I have never run into this glitch before. So I exited to the main menu and tried again. Same thing happened. Exited again, chose a different character, same thing.

I wish the screenshot was better, sorry! This was my last attempt before restarting the game. As you can see I'm Bill, and AI Francis is beside me, making no attempt to help Louis or Zoey. Though as the saying goes, you can't help those who can't help themselves ;P

Fortunately, restarting the game fixed the issue. Still, for all the idiotic AI moments I've experienced, this one takes the cake. Anyone else ever experience this?