On 30 April 1992, the Simpsons episode "Three Men and a Comic Book" was aired for the third time on Fox, up against the last ever episode of The Cosby Show -- and hence, presumably, it might be an overstatement to say that practically no-one watched it (the Cosby finale drawing an audience of 44.4m viewers), but surely it would've attracted a significantly smaller audience than usual, especially since it started halfway into its hour-long opposition. However, the repeat featured a new scene paying tribute to Our Favourite Family's rival (the decision to shift The Simpsons into direct competition with Cosby for its second season in 1990 generating great controversy) at the end (between the executive producer screen and the rest of the credits).
It seemed lucky enough to me that one copy of this scene survived... but it seemed amazing to me to find a second, clearly different off-air of it. As I thought about it, though, it seemed less unlikely -- it's quite possible, if not probable, that the addition of the Cosby tribute would've been trailed in advance. And even back then fans would've known to hold on to something unique like that (even if not every rarity from the show's early years is so fortunate).