MV Sewol Ferry Disaster
In the wake of the disaster, XY024 was postponed (at the
time indefinitely) due to it’s subject matter. The episode, originally
scheduled to air in Japan a mere week after the disaster, sees the main
characters exploring the shipwreck of a luxury liner on the ocean floor.
The episode eventually made it to air in Japan on November 20th.
Source: Bulbapedia