For those of you who ever got this one while invoking javac an a fully compliant java source file, it comes from the encoding of the source not being the one used by your compiler. For example, compiling a source file with comment written in french (and so french special chars) and compiling it under an OS being in US ASCII by default.
The solution is to provide the encoding to use on the command line, like this:
US-prompt> javac -encoding ISO-8859-15 MyFrenchFile.java
This has also come up for me compiling on Fedora Core 2 (FC2). An alternate approach is to change the system's default encoding settings. For me this was to change the following variables in /etc/sysconfig/i18n: