In some fanfiction, Ginny might cheat on Harry because of miscommunication. For example, Harry is often away on Auror business, and Ginny might feel neglected. A new person comes into her life who gives her the attention she craves, and she makes a bad decision. But it's important to note that this is a departure from the loving relationship they had in the books.
One possible scenario could be that Ginny meets an old acquaintance who has changed a great deal and she starts to have feelings for this person while still being with Harry. Maybe they share a common interest that she feels Harry doesn't understand. However, this goes against Ginny's strong character in the original series, so it's a very different take in fanfiction.
There could be a scenario where Ginny is under the influence of a spell or a dark magic object in the fanfiction. This object makes her act out of character and be unfaithful to Harry. This would be a very dark and complex plot twist, as it not only affects their relationship but also delves into the world of dark magic and how it can corrupt even the strongest of characters.
Chapter 73, "Does Someone Like Jiang Li Deserve to Have a Fan Club?", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event.
Chapter 345, "Luo Ming, Mirror Valley Defense Commander, Beast Tide, Great Xia's 6 Armies", is the clearest early answer because it turns Mirror Valley Defense's situation into a concrete plot event. The setup is Great Xia, Year Seven, September Ninth, the latter half of the night..., but the pressure keeps building through For those stationed outside the city and within the Great Xia....
Chapter 315, "New Policies for Long Mountain, Cold Apple Tree and the Silver Mine (Part 3)", is the clearest early answer because it turns Xia Hong's situation into a concrete plot event. A useful checkpoint is the way the scene starts with At present, even the Cloud Dragon Army that cannot appear during the... and follows through with Given that her ages with the eldest and second elder were over thirty....
Chapter 335, "Songyang Ridge, Seven Clans Submit, First Shipment of Silver Arrives", is the clearest early answer because it pushes Ji Yang into a new phase or setting. The setup is In the past, getting married to a lord was wholly different from..., but the pressure keeps building through He exchanged a glance with Nie Xinchuan, a hint of hesitation....
Chapter 95, "The Problem Lies with Your Boyfriend", is the clearest early answer because it turns Xu Ling's situation into a concrete plot event. The chapter turns that into something concrete by moving from Hearing others praise her daughter, Aunt Lin was smiling so wide she...
Chapter 281, "Yang Li's Repression and Resentment", is the clearest early answer because it turns Long Mountain's situation into a concrete plot event. It opens with 48th Year of Longyou, June 1st On the snowy night of Long Mountain,... and closes on In the team, a young man with an honest face spoke up, helping Yang.... The key payoff is that the chapter closes with "Yang Gan and Li Yuanqing sent you to join the team probably to monitor me and Brother Hu, not to ruin my Hunting Team.
Chapter 315, "The Strange Cold Apple Tree and the Seized Silver Mine (3)", is the clearest early answer because it turns Cold Beasts's situation into a concrete plot event. The chapter turns that into something concrete by moving from Thank you, Leader, for the Spirit Pill. to This is almost unquestionable; otherwise, where did so many Cold....
Chapter 343, "Five Cycles of Bone Tempering, 1,000 Zong Bone Burning in the Visible Yang Realm (Part 2)", is the clearest early answer because Xia Hong finally turns buildup into a real advancement. The chapter gives readers a real answer by opening on Next is the Small Ancestral Spirit Tablet, which not only adds a... and ending on But to his utter surprise, by this time it was already too late!.
Chapter 27, "Signing Jiang Yan is fine, but you have to follow along too.", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from They had spent an entire year gradually inviting a few people.
Chapter 100, "Ominous, there might be a bloody disaster!", is the clearest early answer because it turns Ominous's situation into a concrete plot event. The chapter gives readers a real answer by opening on The barrage was about to die of laughter.