facet_grid(a ~ a) produces an error

[vnijs]

FYI The example from #1153 now produces the error below and does not create a plot

ggplot(data.frame(a = letters[1:3], y = rnorm(3))) + geom_point(aes(y, y)) + facet_grid(a~a)

> Error in scale_apply(layer_data, x_vars, "train", SCALE_X, x_scales) : 

[hadley]

@thomasp85 can you please take a look? I don't think we need to make this work, but it should give a more informative error message.