Improper \hyphenation will be flushed

For example

! Improper \hyphenation will be flushed.
\'#1->{
       \accent 19 #1}
<*> \hyphenation{Ji-m\'e
                        -nez}

! Improper \hyphenation will be flushed.
\leavevmode ->\unhbox 
                      \voidb@x 
<*> \hyphenation{Ji-m\'e
                        -nez}

As mentioned in "hyphenation failures", words with accents in them may not be hyphenated. As a result, any such word is deemed improper in a \hyphenation command.

The solution is to use a font that contains the character in question, and to express the \hyphenation command in terms of that character; this "hides" the accent from the hyphenation mechanisms. LaTeX users can be achieved this by use of the fontenc package (part of the LaTeX distribution). If you select an 8-bit font with the package, as in \usepackage[T1]{fontenc}, accented-letter commands such as the \'e in \hyphenation{Ji-m\'e-nez} automatically become the single accented character by the time the hyphenation gets a to look at it.