Solid

I agree that it is somewhat confusing that the marker highlighted is not the 'culprit'. However, it is the marker that could not be placed in the structure - for whatever reason. I don't think Solid is smart enough to figure out what the cause of the error is - it simply states what it couldn't do.

Jon: I agree that SOLID shouldn't try to be that smart, but that's why I suggested simply making both markers red. This reminds the user to think about both of them, rather than only about the second one, since the first is often the culprit. If you don't like making the first one red, you could make it orange or pink or something. The orange/pink font would mean "SOLID doesn't have a problem with this field, but it may be causing the following field to be invalid."

Well, I guess it depends somewhat on who the audience is. I have gotten accustomed in other domains to looking higher up for an error that is flagged lower down. I think this is something SOLID users can be taught. Often it's not the immediately preceding tag that is the problem; it could be something even earlier. I think coloring the preceding one could be misleading sometimes, and could better be handled just by education. Maybe a movie could even be developed to demonstrate how this works, and some of the principles to use for tracking down bugs like this. (Someday, or by a user who thinks it would be a good idea...)