The Digital Hazard symbol appears on the bodies of certain Digimon. According to Digimon Tamers, the presence of this symbol means that the Digimon has the potential to be highly destructive, and, if corrupted, could threaten the very existence of both the Digital World and the Human World. The symbol bears a striking similarity to that which indicates a biohazard.
Digital Monster X-Evolution
When the Digital World reached its maximum data capacity, the host computer Yggdrasill was no longer able to manage the data, and enacted Project Ark to reduce the data load. This overpopulation event was classified as a Digital Hazard by Yggdrasill and his Royal Knights.
Yu's first hat bears the Digital Hazard.
List of Digimon with the Digital Hazard