Anybody still remember when all the adults thought the Snuffalupagus character was only Big Bird's imaginary friend? For the longest while, none of the grown ups ever saw Snuffy; he'd always amble away for some reason whenever Big Bird would excitedly run off to grab someone to meet him.
("Oh, dear...I forgot [insert excuse here]..." and away Snuffy would go.)
Poor Bird would come running back with Maria, or whoever, only to find Snuffy gone. With patience, whichever adult Big Bird roped in at the time would explain that Snuffy was only a figment of Big Bird's imagination, and that Snuffelupaguses don't exist.
Big Bird would adamantly argue, but to no avail. It got to the point where we thought they'd never see Snuffy. (and quite frankly, we, too might have started questioning Big Bird's sanity!)
Then of course, one day the big reveal did happen. All the grown ups finally got to see Snuffy for the first time. They were in shock and awe, and even apologetic to B.B., if I recall correctly. It was one of Big Bird's most triumphant moments on Sesame Street, I think!