The big difference in your example, ferrari, is that the inside ticket is being purchased after the commencement of travel. If the return on the original ticket cannot be changed, then it is appropriate to by a nested return, and that's perfectly kosher.

Another clean example is third city nesting. In fact, this is the basis of the "hidden city" rule in international tariff construction.

Nesting, itself, does not violate the ticketing rules per se. What violates the ticketing rules is back to back ticketing, where you use the nesting to avoid the higher fare rules applicable to the true routing.