Mackie D2 uses it too:
Not sure what fails.
Both times I had problems it was the same thing: moving the crossfader completely to the left made the output value (and graphic) jump back to the centre.
Could be physical I suppose. It's optical technology so maybe the lens assembly (the bit you move) ends up being allowed to travel slightly further than it should and therefore goes out of range.
Who knows?
(That's a rhetorical question)