A temporal rift encountered by the Enterprise-D in 2366 that carried the Enterprise-C forward from 2344 and replaced history with a timeline in which the Federation was losing a long Klingon war.
The Enterprise-C temporal rift is the central event of the Next Generation episode Yesterday's Enterprise. During the battle at Narendra III in 2344, weapons fire opens a rift that throws the USS Enterprise-C twenty-two years forward, where it meets the Enterprise-D.
Because the older ship never completed its defence of the Klingon outpost, history changes at once. The Enterprise-D becomes a warship, Tasha Yar is alive at tactical, and the Federation is close to defeat after two decades of war. Only Guinan senses that something is wrong, and she persuades Picard that the Enterprise-C must go back.
Captain Rachel Garrett is killed in a Klingon attack, and Lieutenant Richard Castillo takes the ship home with Yar aboard, since she has learned that she died a pointless death in the proper timeline. The rift closes and normal history returns.