This might be a weird idea but

What if Sans’s telescope prank wasn’t only a prank, but also a test?

Normally, the kind of prank where you trick someone into getting a mark on their face is a stealth one. The embarrassment is through them not realising the mark and going on with their lives until someone else decides to tell them, or they look in a mirror eventually. They also realise they were the ones who participated, unknowingly, in getting a silly mark.

In this scenario, everyone basically yells SANS and tries to call him on it, resulting in him joking about a refund. But there’s no way Frisk would know about the eye blotch. The normal run of the joke would be for Frisk to obliviously carry on until they find a mirror somewhere. Instead, most people confront Sans immediately.

Maybe he was testing to see if we, the player, really were there. Frisk could’ve turned on him and called him on the prank… despite having no way to know they’ve got a pink eye. That’s what WE know. We outed our existence to Sans.