and who knows what else there might be

U+FFFE and U+FFFF are invalid.

>perl -e"print qq{\xFE\xFF}" | perl -e"binmode STDIN, ':encoding(UTF-1
+6le)'; <>"
UTF-16LE:Unicode character fffe is illegal at -e line 1.

>perl -e"print qq{\xFF\xFF}" | perl -e"binmode STDIN, ':encoding(UTF-1
+6le)'; <>"
UTF-16LE:Unicode character ffff is illegal at -e line 1.
[download]

Same for UCS-2.

There could be more.