Originally Posted by
mozilla
To add to mduell's answer, the discounted/"special" ORDCDG fares have routing restrictions, they can only be used on a list of "legal" routes. You can see the list if you have an EF account, but basically, none of the routes allows MCO as an intermediary point.
While true, that's only tangentially relevant to why an itinerary like OP's used to work but no longer does anymore. United used to allow you to combine cheap domestic fares end-on-end with most of their discount international fares, so
raehl311 would have previously been able to buy an ORD->CDG->MCO open jaw fare and tack on a later MCO->ORD domestic leg on the same ticket. That is no longer the case as United has added significant restrictions to combinability.