Encoding menu won't help. When changing encoding from menu, Notepad not change encoding itself, but try to re-encode file from ansi to UCS-2 again. It's wrong because file already in utf-16. Same bug make impossible open file what start from 0xFF 0xFE bytes. Such combination is valid in some not English 8-bit encodings.
Here two example file: http://rghost.net/41651129
demo1.txt - contains "abcdefghijklmnopqrstuvwxyz" in UCS-2 Big Endian encoding without BOM
demo2.txt - contains "яюэьыъщшчцхфутср" in Cyrillic Window-1251 encoding
Notepad++ unable to open them both