(covers information from several alternate timelines)
A quantum incursion is an event wherein an object or person from one quantum reality gains access to another.
A series of quantum incursions occurred in 2370, when Lieutenant Worf traversed a quantum fissure in a shuttlecraft, throwing him into a state of quantum flux. Whenever Worf entered the influence of the subspace field pulse utilized by Commander La Forge's VISOR, Worf was pushed into a another quantum universe. Later, due to an energy surge in a subspace differential pulse the Enterprise crew was using to scan the fissure, the barriers between realities began to break down, causing approximately 285,000 quantum incursions of multiple versions of the Enterprise. (TNG: "Parallels")