In recent pulls of mupdf, many — perhaps most — horizontal and vertical rules which are rendered black by other pdf interpreters (I’ve tested xpdf, poppler and gs) are instead rendered in various colours.

Red and what appears to be a dark yellow seem to be the most common colours.

Within a given pdf, one colour seems to get used for all of the rules.

I notice this with most of the (recent) pdfs in ArXiV.  An example is:

  http://arxiv.org/pdf/0710.1435v3.pdf

LaTeX uses horizontal rules for such things as the division bar in display equations, the rule above the contents of a radical, and stroked boxes.

In the above pdf, the rules happen to render in red in my release build.

Using a debug build, the colours vary w/in a single document.

I presume, since the two builds have different incorrect results, that the bug is an array overrun, or something similar.

I’ll followup if I can track it down more narrowly.