Created attachment 68390 [details]
harfbuzz-ng (wrong; left) and old pango (proper; right)
Hmm, just spotted a pretty bad rendering bug while testing out firefox linux's use of harfbuzz-ng as a replacement of pango (in change committed Oct 9).
Harfbuzz-ng misplaces and add unwanted duplicate glyphs when rendering Khmer script using Kh-Battambang.ttf, while pango renders the same Khmer script correctly.
I've created a test case here: file:///home/webmaster/Desktop/testcase-refine.html -- you can download the font on the page, as well as retrieve Khmer script linked to this issue (and a <img> of proper rendering).
This bug also contains a screenshot which I believe will be easier to understand than my description ;)
Err, proper test case URL: http://licadho-cambodia.org/mapnik/testcase-refine.html
This is all fixed now.