It turns out source described below probably is not a root source, but may very well be a direct source of the text on your plaqe.
The only instance I can find that's not related to merch comes from D&D reddit:
Specifically this comment posted on Oct 31, 2018. 08:23:15 GMT
Ashenrohk DM 2yr
*player rolls nat 1 on perception*
DM: "You think you're in a room, but you've been wrong before"
It looks weird for this little comment to be source of all this merch, but it's in a thread dedicated to collecting ideas for quotes to put on rpg gadgets. Looks quite probable that gadget manufacturers have found it. Especially given that I (and Medix2) can't find any merch that would actually predate this comment.
According to Carcer
before anyone else bothers them, I have already asked them and they said they definitely didn't come up with it but don't remember where they got it from.
Thus, it looks like it ended on the merch because it was on the "give me merch ideas" reddit thread, but the root source would forever stay a mystery.