In the Minecraft 1.8 (not sure about other affected versions) the activator rails seem to look for an adjacent air block along the x-coordinates to eject entities into. I haven't looked into MCP yet, but I would assume that either the z-coordinate isn't being checked properly or the y-coordinate is being checked next. In that case the mob is ejected onto the rail rather than beside it and is then instantly picked up after that. I was able to reproduce this bug while the mobs were being ejected to a block where the z-coordinate was changed, but if the change in displacement was in the x-coordinate then the mob would be ejected properly.
Mike Doan
In the Minecraft 1.8 (not sure about other affected versions) the activator rails seem to look for an adjacent air block along the x-coordinates to eject entities into. I haven't looked into MCP yet, but I would assume that either the z-coordinate isn't being checked properly or the y-coordinate is being checked next. In that case the mob is ejected onto the rail rather than beside it and is then instantly picked up after that. I was able to reproduce this bug while the mobs were being ejected to a block where the z-coordinate was changed, but if the change in displacement was in the x-coordinate then the mob would be ejected properly.