WBC (wrong but consistent)

At my middle school placement and now back at my high school placement I have been grading tests where problems build upon specific questions, such as slope.  What to do if a student miscalculates slope but then uses this wrong slope to continue calculations.  I adopted the notation "WBC" to mark the problem "wrong but consistent" and I give credit for consistent use of a wrong value.  This system does cause me concerns.  What if you switch the x and y coordinates when calculating slope, a simple mistake, and then I ask you to calculate your y intercept based on an equation using this slope?  In real cases this week I had kids do this.  The problem involved a line of best fit they had drawn so many were aware that the y intercept they were calculating was way wrong.  Some fudged the algebra to make it work, others simply offered the wrong answer, perhaps oblivious to the inconsistency of the two answers.  I wanted to give extra credit to the few who gave the wrong answer with question marks and arrows back to the graph.  I felt like these kids should get extra credit for their understanding of the inconsistency of their answer.  I think they demonstrated a real understanding of the underlying concepts involved.