I'll explain this by referring two people as Player A (also referred as me or I) and Player B.
Player B had went afk beside a wall with a 2x1x2 iron bar wall one top of it. It formed a small, <50cm ledge where the iron bar wall met the wooden wall underneath (sorry if I'm confusing you, pictures below).
While jumping around the confined space in boredom, I jumped onto the ledge. I was in a position where I was sandwiched between the iron bar wall, the wall beneath it, and the player's hitbox. The player stood a block in front and a block below me.
I had sound on, and I noticed something peculiar- the footstep sound didn't turn off. It just kept on looping without variation, making a wood-footstep noise. Nothing turned it off, such as shifting or moving. It just kept on sounding while I was in contact with the iron bar hitbox and that of Player B's. When Player B moved away, the footstep sound instantly stopped.
This error occurred on a vanilla minecraft server with no mods or plugins. Nobody was in the area at the time, and my client is not modified, either.