I haven't seen very many things where Snuffy is said to be imaginary and comes close to being seen.
However, It sometimes seems like him not being seen is forced. In the book Big Bird gets Lost, on the two-page spread at the end, everybody is at the party that Big Bird threw. However, Snuffys foot can be seen coming out from under the table getting some food, and Big Bird asked Snuffy if he was going to join the party.
Here are some close calls that I know of (but haven't seen):
*In one episode, Forgetful Jones saw Snuffy, and Big Bird saw him see Snuffy, but after Snuffy left Bob entered and Big Bird told Forgetful Jones that he saw snuffy, but Forgetful Jones obviously forgot about seeing Snuffy.
*There was once an episode or skit where Bob is sleeping outside, and Snuffy is wearing black and white striped pajamas (I wonder how they were able to get the outfit to fit on him) and Bob woke up just when Snuffy left and told Big Bird and started describing him. However, after he mentioned that Snuffy had black and white stripes Big Bird (who didn't know that Snuffy was wearing pajamas) thought that Bob was imagining things.
*according to the website
www.jumptheshark.com, there was once an episode where Snuffy was right behind Linda, who is deaf.
*there was an episode where Snuffy needed his toaster to be fixed, so he brought it to the fix it shop at a time when maria was too busy to look at Snuffy, so she told Snuffy to leave his name and pick it up later. She told Olivia about Snuffy bringign his toaster to the fix-it shop but Olivia thought Big Bird was just playing a trick on her, so they decided to wait for Snuffy to pick his toaster up, but Snuffy asked Big Bird to pick it up for him, and maria was convinced that Big Bird was just playing a trick.