The Memory Overwriting Machine is a machine located at The Guest House, designed to implant false memories on a subject.
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The machine was used to give the first six test subjects new memories, to ensure that they would lead a normal life. However, they could not go back to being agents.
When Agent Phil Coulson was killed by Loki just before the Battle of New York, Director Nick Fury ordered a group of S.H.I.E.L.D. scientists to resurrect Coulson. As described by Doctor Streiten, who was brought during the seventh operation, the procedures used to resurrect Coulson were ethically questionable, and the trauma he endured was "ungodly".
To overcome those procedures, the medical team led by Streiten kept Coulson conscious to monitor his brain activity, but after determining the high neurological damage he had suffered, they used the machine to give him a pleasant memory of a recovery in Tahiti after his death and resurrection.