I am fairly new to speedrunning this game and play it on a Chromebook emulator. When I was in 4-2 I missed the warp-zone then I got to a warpzone to 5-1. I somehow cliped through the ground third goomba set. I don't know if it is useful but here it is.

Could you record a video of it? By the way, online emulators aren't allowed for actual runs on the leaderboard.

Those kinds of clips happen all the time on the PAL (European) version of the game, could you check if it says (E),(U),(J),(PAL),(USA),(NTSC) etc... in the name on the ROM you are playing on? If it says (PAL), (E), or (EU) then thats probably the reason why, though i dont know why these clips happen.

Also for speedrunning you should get an ntsc rom

yea i have an (E) version

that explains it. Europe version always has weird clips and things due to the fact it was remade to suit the 50fps on european tvs. you should definitely consider getting a NTSC rom if you have intentions of speedrunning SMB 1.

To add onto @Allenbot 's post, I'm pretty sure Mario himself technically moves faster on PAL than NTSC, which causes clips like that and Full FPG on 1-1.

If I'm wrong then someone can correct me.