A bacteria created hole in the hull of the Pagh.
Subatomic bacteria is a type of bacteria that was found feeding on the tritanium plating of the IKS Pagh's hull.
Following its discovery, Klingon Captain Kargan believed it was part of a Federation plot. However, the subatomic bacteria was also discovered on the hull of the USS Enterprise-D by Mendon.
It was safely removed from both vessels via the use of a tunneling neutrino beam. (TNG: "A Matter Of Honor")