Really? I was able to reproduce it twice using mirror mode; but not very consistently.
It seems that after teleporting, the client sometimes retains the old X coordinate of the player from before the teleport; but only until properly refreshed (by relogging, enabling player debug, or even just waiting). I don't quite have enough information to report it as a client bug yet, since I can't reliably reproduce it.
Does this happen to you after relogging in the start location; and/or does it consistently happen when trying again?