GitHub satanically messing with Markdown – changes 666 to DCLXVI
Reddit also messes with numbers followed by periods at the start of lines. It doesn't convert them to roman numerals, but it does infer from their presence that you are trying to make a numbered list, and it apparently also assumes that you have no idea how to count, so it renumbers the list for you.This regularly screws up posts in /r/chess. Someone might post something like this:  1.e4 e5 2.f4 exf4 3.Nf3 g5 4.Bc4 g4
  5.O-O gxf3 6.Bxf7+ Kxf7 7.Qxf3 Bc5+
  8.Kh1 d6 9.c3 Nc6 10.d4 Nxd4 11.cxd4 Bxd4
  12.Bxf4 Nf6 13.Bg5 Rg8 14.Qh5+ Rg6

and Reddit changes it to this:  1.e4 e5 2.f4 exf4 3.Nf3 g5 4.Bc4 g4
  2.O-O gxf3 6.Bxf7+ Kxf7 7.Qxf3 Bc5+
  3.Kh1 d6 9.c3 Nc6 10.d4 Nxd4 11.cxd4 Bxd4
  4.Bxf4 Nf6 13.Bg5 Rg8 14.Qh5+ Rg6