A Quantum Fissure is a fixed point in the space-time continuum opening a doorway into other quantum realities. A person or object entering a quantum fissure will shift to different realities randomly.
In 2370, Lieutenant Worf while returning from a Bat'leth tournament on Forcas III encounters a Quantum Fissure and the shuttlecraft's engines caused a small break between the quantum realities. Worf was thrown into a state of quantum flux and immediately shifted into other quantum realities. A primary cause of Worf's shifts were due to Geordi LaForge's VISOR emitting a subspace pulse which intensified the quantum flux throwing Worf into another Quantum Reality. (TNG: "Parallels")