Even long after memories had been erased, a Memory Charm could still be reversed. Even the Cruciatus Curse could restore memories—in a sense, it could be called the Wizarding World Great Memory Restoration Spell, except that this method damaged the soul.
Will knew of a wizarding official named Bertha Jorkins. While holding an important post in 1980, she had discovered that Barty Crouch Sr. was hiding Barty Crouch Jr., and the powerful Memory Charm the elder Crouch cast on her had caused permanent damage to her memory.
While vacationing in Albania in the summer of 1994, she had been lured into a trap by Peter Pettigrew and handed over to Voldemort. Under the agony of the Cruciatus Curse, she had remembered that secret, leading to the events at the beginning of the Goblet of Fire and everything involving Barty Crouch Jr.
Now, however, Voldemort had not traveled all the way to Albania. Instead, he was staying comfortably in England, so it was hard to say whether anything would happen at the Quidditch World Cup.
Still, Will did not want any major gaps in Peter's memory to expose him, so he would have to be a little more precise...
Peter groggily opened his eyes and saw the familiar underside of the bed. His head felt fuzzy, as though he had slept too long. His stomach growled, making him want to get up and find a couple of snacks from Ron's stash.
A muffled thud! rang through the dormitory. Fortunately, classes were in session, so no one noticed.
Only then did Peter realize that he was actually in human form. He expertly turned into a rat and recalled what he had done yesterday, only to conclude that he had simply slept too long—apart from sneaking Ron's snacks, all he had done yesterday