My reference list include letters from different languages, such as á, ö, 第.
I get
! Package inputenc Error: Keyboard character used is undefined
EDIT
[Copied from deleted answer by cfr as the OP cannot currently access the account.]
Augustin's suggestion solved part of the problem. It still do not want to work with 第 (and the likes).
! Package inputenc Error: Unicode char \u8:第 not set up for use with LaTeX.
I did try to, as per one of the answers, create a new document and try again. It did not work.
Would still be nice to know how the get around it, though. Might need it a rainy day.
The problem was with a reference I wanted to include an alternative title for.
– BinarySolo Nov 15 '15 at 21:38\bibitem{key-12} something 京都议定书, 1997.