The spec on java.nio.file.Files.newBufferedWriter states:

"The Writer methods to write text throw IOException if the text cannot be encoded using the specified charset."

Actually write methods do not guarantie to fail in that case: they only fail when flushing to the underlying writer. For example:

BufferedWriter wr = Files.newBufferedWriter(path, Charset.forName("US-ASCII"));
wr.write("\u00ff"); // does NOT throw IOE
wr.flush(); // or wr.close() throws IOE

The spec should clarify that behavior.