Sometimes one may post a question to a particular SE site, and then they may realize (possibly with help of a commenter) that this question better fits other SE site, so the poster deletes their question and posts it to the other site.
While this is happening, other users may have been writing an answer, and after pressing [answer] button, they get question was deleted by poster message, though their answer may have been useful to the poster. Usually people don't go looking where the poster may have moved his question and the effort is wasted, but in some cases (advances users) they do and they are good people for doing that :)
I think this should be solved by the site itself. For example provide a tip that suggests to look at similar questions recently posted by the user, including other SE sites.
One other way is to let the user who deletes their question mark a checkbox "I intend to post this deleted question on another SE site", preferrably with an option to later link to it, so users who may have already typed their answers post them there.