I've reported about this possible bug already in 2010, and I believe it has
not been fixed since, can someone please confirm this and reproduce the
error?
Problem arises when setting the mainfont using font disfiles as shown
below. Will Robertson commented in 2010 that this is likely a bug...
\usepackage{fontspec}
\usepackage{xunicode}
\usepackage{xltxtra}
\usepackage{polyglossia}
\setmainlanguage{english}
\setmainfont[Path=C:/fonts/SabonNext/,
BoldFont=SabonNextLT-Demi.otf,
ItalicFont=SabonNextLT-Italic.otf,
BoldItalicFont=SabonNextLT-DemiItalic.otf]
{SabonNextLT-Regular.otf} % Sabon next
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