Weird bugs with relative teleportation in SMP

I cannpt destribe this bug with words, as it couldn't be replicated last time, so I made a video about it.
Commands I used:

 /execute @a ~ ~ ~ detect ~ ~ ~ activator_rail 0 tp @p ~0.4 ~ ~ 
 /execute @a ~ ~ ~ detect ~ ~ ~ activator_rail 1 tp @p ~ ~ ~0.4

on a 20 times a sec clock
http://youtu.be/hiEd7F9uTJg
EDIT: This DOES work in Singleplayer but it is bugged in Multiplayer!