well attempting to ride ether a horse or a boat in to a area where a repeat command block (or any) is set to use the spread players command it will bug out resulting in the player being tp'ed but the horse will not and the player will not appeared to have moved on there own screen and they can still control the horse and can not demount (this is well using the @a for the selector)
now if you use @e it gets a bit worse as the player will not teleport but the horse or boat will and the player will sometimes move with it but chunks will not load/render at all and using the debug f3+a to reload chunks makes it worse and wane using just f3 it will state it is trying to find the chunk
English please