The following are features that a proper void lane handler should have:

• Players that exit their lane should not be killed immediately if they could fall out of the world.
• Players that re-enter their lane after exiting their lane should be killed immediately, with no items dropped.
• Leniency should be shown in cases where the lane is only briefly exited if it is re-entered in the same location.
• Death messages regarding the void lane should be shown exactly once (issue #3)

Has this been tested in pain water in DF? I know you can hang in water while 'not being in it' (you can't drown if you swim at the side and yet you don't fall). I'm not sure about what your coordinates are at that moment.