I probably remember it wrong then. The situation is as follows: Person A has written a page and marked it for translation, Person B translates it. Now Person A adds some text, but Person B can't translate the new text, because the new version hasn't been marked for translation yet.
So the "It says the translation is outdated" part is probably not correct, the question is why a translator sometimes can't update a translation. (A: The new version hasn't been marked for translation.)
I think you are mistaken. As far as I know the translators only see messages that refer to the version already marked for translation. It's possible to have a development version of a page for some time that does not get marked for translation until the author clears it. In that case the translator sees nothing about it.
OK. How about a short note then? The situation described above actually happened during the Plasma documentation writing.