In the following example, the PDF compiles without error using xelatex:
\documentclass{article}
\usepackage{fontspec}
\usepackage{xunicode}
\usepackage{polyglossia}
\setdefaultlanguage{english}
\begin{document}
Hello there. Do you know ?
\end{document}
The character, however, is missing in the PDF.
The log file shows:
Missing character: There is no in font [lmroman10-regular]:mapping=tex-text!
Why is this a warning and not an error?