FWIW - I get these errors when the data buffer fills up because the grid/arc refresh rate is too fast.
In the case of my DIY grid and DIY arc - this is about 30 frame per second (fps). Anything faster will throw libmonome errors (with certain scripts). Real Grids/Arcs seem to do ok up to 40/60 fps from what I understand.
However, step is checking to see if the grid is dirty before refreshing, so I think that means its not actually calling grid:refresh() every 1/60th of a second, right?
EDIT - for example - Glut (line 287) will throw these errors for me at 1/40, but not at 1/30. Crank that number up (down?) and you can probably reproduce the error.