\@noitemerr macro that generates this error is used in the list code for some cases of possibly empty lists but (for reasons lost in the mists of time) it is also used if a vertical space is added when you are in horizontal rather than vertical mode. This is by far the most common cause of the error.
! LaTeX Error: Something's wrong--perhaps a missing \item.
See the LaTeX manual or LaTeX Companion for explanation.
Type H <return> for immediate help.
See also the tex faq entry for this error: