Funny, i broke my Debian twice (on two separate laptops) by doing exactly that, mixing stable with testing. I was kinda obliged to use "testing" because Dell XPS would miss critical drivers.
In fairness, this is the number one way listed [0] on how to break Debian. That said, if you need testing (which isn’t that uncommon for personal use; Debian is slow to roll out changes, favoring stability), then running pure Sid is actually a viable option. It’s quite stable, despite its name.
I switched to opensuse afterwards