So for season 16, Gordon, Maria, and Linda all believed in Mr. Snuffleupagus, even though they wouldn't meet him for a year.
Until the trusted sources guides came to the wiki, it seems like none of the fans had any episodes that mentioned them believing in Snuffy (with the exception of episode 2096, where it's mentioned a few times that they already believe Big Bird). There was at least one fan who had the season 16 episode with Snuffy joining the Grouchkateers, establishing that Oscar knows he's real, but otherwise, I wonder if there would have been confusion by fans seeing one of the season 16 episodes where one of them is shown believing in Snuffy. Though maybe it wouldn't seem weirder than episode 1836 having a couple scenes where Maria is willing to see that Snuffy is real, or the later season 15 episode where Maria and Linda happily make get well soon cards for Snuffy even though they think he's imaginary.
There was also a season 16 article about this new development, which was sourced by the wiki before the trusted sources came. But how well known was that article to us (well, if it was better known, we'd have likely gotten details on episode 1966 a lot sooner)? Of course I recall seeing somebody on this forum talk about that episode, before the wiki even got started, and there were fans who have remembered the episode where Oscar first sees Snuffy,
And it was noted on the wiki that Uncle Wally believed that Snuffy was real before the rest did, but then I saw in a recent edit description that it turned out he was just humoring Big Bird (that guide hasn't been made yet, so I don't know if they were going by the script or video footage), which would make sense that in episode 2096 they never mention Uncle Wally believing Big Bird (though if he really didn't, he is the only one who didn't who disagreed that "food" is a silly secret word).
The wiki still doesn't have a guide for the episode where Oscar first met Snuffy but it is said that it's episode 2042, so 53 episodes before everybody met him. I don't know what the dynamic was (if Oscar told them and they didn't believe him, or if he refused to tell everybody, or if it just never came up), but it is interesting how in episode 2096, it's never mentioned in his one scene that he had already seen Snuffy (though he doesn't seem to know what the secret word business is all about). It's noted on the Snuffleupagus Sightings sandbox page that they had actually met off-screen in a season 9 episode, which I questioned after reading the guide. I thought Snuffy stuck his nose out Oscar's can when everybody, including Oscar left, but maybe Oscar tried to tell them there was one more in there or maybe they were heard talking (the guides don't have to be complete on all segment details). Though if they were definitely heard interacting, this is a weird continuity detail to not keep track of (even if they wouldn't interact again for seven years), not to mention an odd thing to just be a small part of an episode with no major developing.
I guess this - the stuff about some of the adults believing in Snuffy in season 16 but hardly any of the episodes were available to us for years - could be similar to Buffy and Cody staying in Oscar's trash can in season 12. When we first saw the "bring your own can night" clip from episode 1553, fans were wondering why Oscar had them all in his can, if we had seen the full episode then we'd probably still be confused as to why Buffy and Cody were in the can already, and none of us had any of the other episodes where they were staying over. When I saw the first guide Muppet Wiki added to reflect this (the Camp Uglyyuccy episode), I instantly put two and two together and figured it out. I also hadn't realized that for the majority of her time as a regular that she was usually just visiting the street for many weeks. I was confused as to why the cast visited her in Hawaii in season 9, not realizing that they were living there full time and just visiting the street for extended periods.