This error occurs anytime you have pipe in caesar that occupies the same space as other pipe, namely elements 10-15 and 10-500.

"Proper" elbow attachment methodology would have you model 10-15-20 as normal, and then include a 14-500, length = 200.

An even more accurate modeling technique would include modeling from 14 to the extrados and attachment CL intersection point (call it 500, and model from 500-520 as you described above. However, this is only about 0.2% difference from "Proper" as described above, according to Dave Diehl's webinar from some 3-4 years back.