What does the warning about "moved marginpar" mean?

Thank you for the answers! I add my own since I found it on the todonotes package author's blog page:

http://midtiby.blogspot.com/2008/05/updated-todonotes-package.html?showComment=1215360900000#c7770492295748791763

Your problem with the "Marinpar in page x moved" is probably caused by placing several todonotes on one page. If two todonotes (they are in fact marginnotes) are goin to overlap, latex will try to move the todonotes up and down on the page to avoid the overlap. To let the user know that the marginpars have been moved, the warning is issued.

In fact, just deactivating the todonotes packages makes the warning go away. This is fine since those notes are only useful for the draft version.

Alternate explanation:

Do you have multiple marginpars? It may be that placed in their natural position they would overlap each other, so one or more were moved a bit to make room.

I've used marginpars for an internal FIXME tool before, and using a lot can result in some being displaced from their associated text.

It has nothing to do with overlapping margin notes. TeX's \marginpar is treated like a float; the page-breaking mechanism is independent of the paragraph layout, so if a paragraph containing a marginpar is broken across pages, the page break can occur before the marginpar gets output, and since it can't back up and output stuff on the previous page, the note ends up at the top of the following page rather than alongside the line where the command occurred; that's when you get the warning. (\usepackage{marginnote} will get you a non-floating \marginnote which solves this problem)