The back corner is something of a red herring, because it's generally the last part we draw. So, as we construct our box, these little mistakes accumulate (that is, the mistakes from our sets of lines not converging completely consistently). They don't show up until we reach this last part of the box however.

This causes students to think, "my back corner is wrong, I better focus on that back corner" - when in fact, the back corner is merely a symptom, and it's actually the convergences of those lines that causes it. So, always focus on the line extensions and what they tell you about how you're approaching drawing each line, and what can perhaps be altered in that approach. As your convergences become more consistent, that back corner will far more in line. But don't fall for its siren call, because it is not the problem, just as your smoke detector is not to blame for your house burning down.