From Bulbapedia, the community-driven Pokémon encyclopedia.
The frozen Zoroark glitch is a glitch that occurs in the Japanese versions of Pokémon Black 2 and White 2. If the game is saved after the glitch is performed, it will severely affect the storyline. This bug was fixed in international releases.
Performing the glitch
After the player beats the Champion, it will be possible to access the ruins of N's Castle, by following N's Zoroark in Victory Road. If said Pokémon is followed, but then the player leaves the area without following Zoroark all the way to the end, Zoroark will reappear in its initial position upon the next visit, and will not move.
This game anomaly causes Zoroark to permanently block the path to N's Castle, preventing the player from meeting N and thus from experiencing other significant events, like the battle with his ZekromB2/ReshiramW2 and with Normal form Kyurem (thus also preventing the player from obtaining the DNA Splicers and the version's game mascot).
*Indicates glitches which have been officially acknowledged