The formation of a subspace rift in the Hekaras Corridor in 2370, deliberately caused by a scientist to prove that warp travel was damaging space, which led to a Federation speed limit.
The Hekaras Corridor rift appears in the Next Generation episode Force of Nature. The corridor is the only safe route through a region of heavy tetryon fields, and the Enterprise-D enters it to look for a missing medical ship, the Fleming.
Two scientists from Hekaras II, the brother and sister Rabal and Serova, have been disabling ships to draw attention to their theory that repeated warp fields are wearing subspace thin. When the Federation asks for more research first, Serova overloads the engines of her own ship. She dies, and the explosion tears open exactly the kind of rift she predicted, with the Fleming caught inside.
The Enterprise rescues the crew by coasting through the rift without warp power. The Federation Council then restricts all ships to warp five except in emergencies, a rule mentioned again in later episodes of the series.