I was affected by bug 487 using microtype's protrusion=true. Digging around, I found that the error lies in round_xn_over_d (pdfgen.w). Please find attached a patch to correct the issue. Details: round_xn_over_d calculates (x*n / d). It casts to unsinged integers, guarding the sign of x but not of n. Negative protrusions, however, are fed into n... Aside: I wonder whether its method to calculate still makes sense, given modern compiler optimizations. What about something along the lines of..? unsinged long long t = x * n; t = t % d; if (t > INT_MAX) arith_error = true; return (scaled) t; Dominic --- a/luatexdir/pdf/pdfgen.w 2014-04-23 10:46:24.485686773 +0200 +++ b/luatexdir/pdf/pdfgen.w 2014-04-23 10:49:03.354059350 +0200 @@ -751,13 +751,15 @@ @c scaled round_xn_over_d(scaled x, int n, unsigned int d) { - boolean positive; /* was |x>=0|? */ + boolean positive = true; /* was |x>=0|, |n>=0| ? */ unsigned t, u, v; /* intermediate quantities */ - if (x >= 0) { - positive = true; - } else { + if (x < 0) { + positive = !positive; x = -(x); - positive = false; + } + if (n < 0) { + positive = !positive; + n = -(n); } t = (unsigned) ((x % 0100000) * n);//printf("t=%d\n",t); u = (unsigned) (((unsigned) (x) / 0100000) * (unsigned) n + (t / 0100000));//printf("u=%d\n",u);