In my SMP server, after players got in the nether portal, staying inside the purple portal blocks would cause constant teleportation between the nether and overworld. Whereas in vanilla Minecraft, one would have to exit the portal then walk back in to once again activate the teleportation. This caused a MAJOR issue for players who's loading screen took too long to load causing them to be either helplessly stuck in the portal teleporting back and forth forever, teleported thousands of blocks in the overworld or stuck in lava and be killed.