From Halopedia, the Halo wiki
A slipspace anomaly is an unusual event that occurs in slipstream space. Several abnormal slipspace behaviors could be categorized as slipspace anomalies. Slipspace anomalies are very rare and can put a craft's ability to transition safely into Slipspace in jeopardy.
One slipspace anomaly occurred in 2552, in which a Forerunner artifact carried by the UNSC Apocalypso created a giant rip in both realspace in Lunar orbit and in Slipspace. The rip acted as a portal between the years 2552 and 2004, sending part of the Apocalypso's shipboard AI construct, Melissa, through it. In addition, all communications within the Sol system were silenced for over seven seconds. When the artifact was deactivated several weeks later, the rip closed and the AI was sent back to 2552.
The Forerunner crystal found in the Menachite portal complex on Reach, coupled with Cortana's incorrect Slipspace calculations also created a Slipspace anomaly. This anomaly caused the ship the artifact was in, as well as all surrounding ships, to enter an anomalous Slipspace dimension when transitioned into Slipspace. One could fire their weaponry in this sub-dimension, but Covenant ships could not recharge their shields. Shots fired did not obey the standard laws of physics and instead disappeared and reappeared in different locations until they hit something. There was also fog present in the bubble, instead of the black nothingness of normal Slipspace.
The Forerunner Dreadnought's wake
On November 3, 2552, after witnessing the aftermath of the Battle of Installation 05 and the beginning of the Great Schism, the crew of the UNSC Dusk set course for Earth. The prowler arrived at Earth within hours by following the Slipspace wake of the Forerunner Dreadnought. Strangely, the Dreadnought did not arrive in the Sol system for a further five days, and it did not arrive at Earth until November 17.
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