Robotaxi companies are learning the hard way that operating driverless cars also means that there’s no one supervising what happens inside them.

Take Zoox, the Amazon-owned self-driving startup that’s been offering rides, free of charge, in Las Vegas and San Francisco since September. It claims that it’s served more than 500,000 passengers — while acknowledging that some of them haven’t exactly been sticklers for the rules.

“You’re not supposed to smoke in the vehicle. They smoke in the vehicle. And they’re smoking everything in the vehicle,” an exasperated Chris Stoffel, Zoox’s director of robot industrial design and studio engineering, told Fast Company in a new interview. “Like, how do we interact with that?”

Unlike its competitors, Zoox uses its own unique car that it designs and builds in-house. There’s no driver seat or steering wheel, let alone traditional car doors. Instead, it has two rows of face-to-face seats fitting four people, and sliding doors that part in the middle. It’s like a train car on wheels, if the train car was way smaller and looked like a minimalist toaster.

The fundamentals of this unique design has stayed the same through the years, but Zoox recently unveiled a new “evolution” of the car, as it prepares to start giving rides to the public in Austin and Miami.

Many of these changes were passenger focused, according to Stoffel, now that it’s had plenty of rides to learn from. And one particularly informative recurring passenger behavior? Vomiting.

“Although we thought about that in the initial product and initial materials, we can make it better, right?” Stoffel told Fast Company. “Because now we have real-world experience saying like, hey, this is how long it took us [to clean], this is where the vomit got into the nooks and crannies. Can we adjust that? Can we fix that?”

Combined with the plague of passengers hot-boxing in their cars, Zoox engineers pivoted to using more moisture and odor-resistant materials. Sayonara, weed smoke and throw-up stench!

 “We wanted to be able to wipe clean any issue very quickly rather than replace parts,” Stoffel said.

Zoox isn’t the only robotaxi company dealing with rowdy riders. Some passengers have reportedly had sex in their self-driving cabs, and most recently, a group of teens made headlines after they were spotted hanging out the windows of a Waymo robotaxi. How these services will keep riders on their best — or at least acceptable — behavior is something we expect to be a major challenge as they grow in popularity.

More on self-driving: Waymo Caught Driving Straight Into Oncoming Traffic