One of my boas does this every single time. So i switched her from a cool looking hide to half of a cereal box, she likes it all the same, it costs me next to nothing, and i can just throw it out when she does
I wish i don't need hides like PMSpikeanoid but i have a terrarium for my bp, that's why it's unavoidable My Kahlua pees in his hide a lot lately. I'm afraid he'd get any infection from that, so i get him washed whenever i have a chance to.
It's not uncommon. Snakes will go wherever, whenever. Ball pythons spend a lot of their time in their hide and if they have to go, they're going to go. I use bark chips for the pythons as it retains humidity well and it's easy to spot clean. I check everyone daily to give them fresh water and if I see anything, remove it and we're good to go.
I use Uline indented kraft paper - love it but it can be pricey. It never fails, I will change out the paper due to a little pee and then bam - huge pooper comes within a day. Mine also will mess while in their hides and then proceed to lay in it. You gotta love em!!!!