[OS X TeX] \inaccessible \bibitem

[Ingo Reich]

Hi everybody,
I just added to my document (memoir class) a new citation (I use  
natbib.sty for citations) which resulted in the following error message:

! Missing control sequence inserted.
<inserted text>
                 \inaccessible
l.149 \bibitem[Sternefeld(2004)]{Sternefeld:04}

?

The funny thing is that when I add another new citation, the error  
message disappears again.
Any ideas what may be the cause of the error message?
(The bibentry seems o.k. and I also tried different bibstyles)
Ingo

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