On Math Stack Exchange (and on Stack Overflow, and presumably other sites as well), there are sometimes questions intended for the main site that are asked on meta instead. This is presumably because the site design is virtually identical except for
Muted colors (which a novice user may not realize are indicative of anything, and which a color-blind person may not notice) and
the word "meta" appearing in the site design (with varying degrees of distinctiveness, or not at all).
While I strongly believe that efforts should be made to further visually distinguish main sites and meta sites (as was proposed in this meta Stack Overflow thread, which inexplicably has an accepted answer), I would like to propose here that when a user is about to post on meta for the first time, an announcement, for example, using placeholder text in the question-writing box,
explain to the user where they are,
ask if that's where they want to be, and
provide an unmistakable link back to the main site, for if they decide they are in the wrong place.
As Mariano says in the post that prompted this one,
Given any such question, it is very easy for a mod to move it to the actual math.SE site, and users with sufficient reppoints can vote to have it moved themselves, but it would be immensely more efficient to fix whatever problem it is that is making people post them here in the first place.