When using a minecart it is possible to get though the worldborder if this is set to anything but a number like 1 or 3 or 5 or 7 and so on.
To replicate: Make a worldborder like "/worldborder set 10" or "/worldborder set 10.5" As long as it is not going to consume the entire block this should work. Then be inside the worldborder, place minecart rains facing into the worldborder and place a minecart, when you hit the border just dismaunt and you may now be outside the worldborder (in some cases it dont work). You are now free to walk as you please, but can still not place or break anything.
If you set the diameter to a number like 11, it is still possible to get outside but you cannot move around, just stay or go back inside.