Cana could be embarrassed when she tries to show off a new card - based magic technique to the younger members of Fairy Tail. But she messes up the incantation and instead of a cool display, she creates a bunch of sparkly but useless card confetti that rains down on her, making her look silly.
In the case of Pearl and Sky, when Sky made a special floating platform for Pearl so she could experience flying for a little while. It was a moment that symbolized their willingness to share their different worlds with each other, and it was a beautiful display of their love.
Think about the themes in Fairy Tail that can be turned into a sad story for your OC. For instance, the concept of friendship. Your OC could be betrayed by a supposed friend in the guild. Develop the OC's character arc in a way that shows their downward spiral into sadness. Show their reactions, like crying alone in their room or avoiding the guild members they used to be close to.
One sad OC (Original Character) fanfic in Fairy Tail could be about an OC who loses their magic abilities. This would be a really tragic story as in the Fairy Tail world, magic is such an important part of one's identity. The character would have to struggle with feelings of worthlessness and isolation.
One sad Nalu story could be about a time when Lucy is in great danger and Natsu is too far away to reach her immediately. He races towards her, fearing the worst, and the sense of impending loss makes it a sad moment. For example, when Lucy is captured by a powerful dark guild and Natsu is on a different mission. He feels helpless and full of self - blame.
The character development. If a fanfiction shows a character's downward spiral in a believable way, it becomes sad. For example, if Happy suddenly loses his ability to fly in a story. His normally cheerful self would be so devastated, and seeing his friends try to cheer him up but fail would be really sad.
A sad Fairy Tail fanfiction could focus on Erza. Maybe she gets trapped in a magical dimension where she is all alone. She has to relive all her worst memories and fears. The guild tries their best to get her back, but she starts to lose her will to fight and the fanfiction could explore her inner turmoil and the sadness of being isolated.
One popular one could be 'The Hilarious Adventures in Fairy Tail Guild'. It has a series of short stories within the fanfiction that focus on the comical interactions between the guild members. Another is 'Fairy Tail's Funniest Pranks' which is all about the various pranks the characters play on each other.
You can try some popular fanfiction websites like FanFiction.net or Archive of Our Own. They usually have a large collection of 'Fairy Tail' fanfictions, including those about funny moments.
One exciting moment could be when Natsu first unleashes his most powerful Fire Dragon Slayer attack against Laxus. The sheer power of the flames and the look of determination on Natsu's face would be thrilling. Another moment might be when Laxus counters with his Lightning Dragon Slayer magic, creating a spectacular display of light and energy as the two elements collide.