I can’t say this with certainty but I suspect this is a similar problem to the line 19 stone placement bug that is being hunted down with some tenacity. This is the bug where a stone will randomly jump to line 19 when the submit button is pressed (or if the move is automatically submitted).
I notice the line 19 error most prevalently after the tablet screen has redrawn to accomodate the keyboard (for example, should I type in the chat window before making a move). I don’t know if this reliably reproduces the problem, but it may.
Kudos for tracking down this particular bug, though.