Quantum-chemical calculations reveal that metallic carbide endohedral fullerene Y2C2@C-84 possesses a novel fullerene cage, C-1(51383)-C-84, with one pair of pentagon adjacencies. One of the encapsulated yttrium atoms is located on the adjacent pentagons, while the other stays on a hexagonal ring in the fullerene cage. As one of numerous metallic carbide endohedral fullerenes, Y2C2@C-1(51383)-C-84 is the first example that violates the well-known isolated pentagon rule (IPR). More interestingly, compared with the fact that Sc2C2@C-84 has a conventional IPR-satisfying cage, D-2d(51591)-C-84, Y2C2@C-84 utilizes the novel fullerene cage C-1(51383)-C-84 with one pair of pentagon adjacencies.