Plot failure: "Visible pixel grid has a scan line longer than previous scan lines"

[Dima K. <gn...@di...>]

Can somebody tell me what this means?

I'm plotting a grid of data as an image. And I have two identical plot
commands, differing only in the data being plotted (attached). One
works, and the other doesn't, throwing the error in the subject. Does
anybody know what this means? It should work...