This may sound controversial, but actually it’s just the logic conclusion to my experience. Also, if you’ve recently read something similar, it’s likely I’ve written it: I’m just trying to spread this as much as possible, because I don’t think it receives the proper attention (from potential customers, but especially from Valve).
Two days ago they delivered my 512 GB refurbished Steam Deck. According to Steam,

Each Certified Refurbished Steam Deck has been thoroughly tested to the same high standards as our retail units. Every device goes through a complete factory reset, software update, and an extensive examination involving over 100 tests at one of Valve’s facilities. Among the tests are all controller inputs, the audio system, the screen, and internals. Battery health is also assessed to ensure proper functionality and longevity.

All refurbished units meet or even exceed the performance standards of new retail units. Although they may have minor cosmetic blemishes, they provide a reliable, high-quality gaming experience at a lower cost.

First game I wanted to play was Skyrim. I’m still on the carriage on the way to Helgen, and the game freezes. Then the screen goes dark, and when it comes back on the game is still frozen and unresponsive: only the audio keeps playing.
Long story short, this issue is known as ring gfx_0.0.0 timeout because of the log message displayed by the system when you run a game through the terminal and this happens; it is a pretty widespread issue, which I don’t think Valve has officially acknowledged just yet.
Currently, I can reproduce this within 10 seconds of actual gaming. By “actual gaming” I basically refer to whatever you get after the graphic engine has been loaded: for instance, with Skyrim it is whatever you get after loading a save file, or starting a new game; with L4D2 it is whatever you get after choosing a map; and so on.
Personally, I think 10 seconds can be considered as “testing,” and maybe one minute could be considered as “thorough testing.” But if anyone knows about refurbishing, I’m happy to listen. So, how exactly could a problem like this, occurring consistently as soon as you run a game, bypass the oh-so-thorough testing refurbished units are supposed to go through? I think now you can see why I said “refurbished units are not thoroughly tested.” Honestly, I would be surprised it they’re tested at all.
Or maybe I just got unlucky, and they shipped the only unit they forgot to test.