Jumping the flagpole in Super Mario Bros. is possible. This has been proven. Have you seen it on World 1-1, though? Well, thanks to this glitch, you will. [NSFW audio because this guy’s nom de plume is ‘Motherfucker Mike’].
https://lastchance.cc/once-and-for-all-mario-can-jump-the-flagpole-452577208%3C/a%3E%3C/p%3E
I don’t know if glitching the game counts as actually leaping it but, yes, there Mario goes, having slipped the surly bonds of 1-1, to put out his hand and touch the face of Miyamoto.
Jumping Over The Flagpole On Stage 1 in Super Mario Bros [Cinemassacre, thanks Manly McBeeferton]