This rule isn't just limited to just big-breasted girls in Negima, as Nodoka, who is a bit of a Pettanko, is seen hiding one of the smaller versions of her Pactio item in her cleavage in chapter 280.
During the sports festival, several girls in class 3-A pull love notes to Negi out of their clevage because they're unable to express their feelings aloud.
Villain Ball: Cosmo Entelecheia attacking the Governor's reception for no explained reason, while they already had all they needed and were supposed to be lying low.
Villainous Rescue: Evangeline saves the gang's collective ass in Kyoto, demonstrating her true power in the process by curb-stomping both a recently-summoned demon god and a later arc's Big Bad in quick succession.
Also often played for laughs when Yue is in a pinch, showcasing her tendency to overanalyze everything.
Waterfall Shower: Done on several occasions, most notably during the descent into the Library in volume 3, and then again much later in the Magical World by Asuna and Setsuna. and the Crowning Moment of Funny when Rakan and Chisame catch each other doing it.
The Watson: Usually the normal or inexperienced girls. The trained warriors are usually the ones explaining.
Weapon Jr.: Small wands are used for magic training and as emergency magical foci. Chamo even describes them as beginner wands.
We Are as Mayflies: The series has a vampire, several robots and demons, all effectively immortal, and at least some of the species of the Magic World (such as the one of Jack Rakan) are said to have a pretty long lifespan.
Well-Intentioned Extremist: Chao, Fate, and Kurt Godel all want to "save the world" through means that others think are going too far.
We Will Not Use Photoshock In The Future: Kosmo Entelekheia edits video of the fight at the Gateport to make it look like Negi's group had destroyed it instead of themselves, causing Negi's group to become wanted criminals. Although it probably doesn't help that there's a Government Conspiracy against them.
Chisamelampshades this early on, when Negi notices how much better she looks in her idol pictures but doesn't put two and two together.
Chapter 277, in which the Big Bad's allies reveal they can simply erase people from existence, and do so to several minor-but-named characters.
Chapter 294: the reveal of Zazie as the group's next opponent. Fans shat bricks, and Chisamelampshaded it in the next chapter. Though it's all undone by revealing that it's not, in fact, Zazie, but her older sister.
(Then again, there's a lot of misleading foreshadowing in that collage, starting with the fact that the size of most of the characters' pictures is inversely proportional to their later role in the story.)
Chapter 311 Quartum rips Chachamaru in half, Quintum steals back the Code of the Lifemaker, so now there are 4 averruncus' one of which is female.
Chapter 331 features the arrival of all the former Ala Rubra members currently accounted for including none other than JACK Freaking RAKAN to join Eva in taking on KE, giving the impression that Akamatsu is out to either give his readers a concussion or just straight out split their skulls open.
Chapter 334 which ends up revealing the current Lifemaker to be Nagi.
Chapter 346 in which Negi reveals that he has feelings for one of the girls (doesn't so which one though)
Chapter 352 alone blows them all out of the water. Asuna wakes up in the future several years late and meets the descendants of several classmates. Furthermore, she finds Negi's grave (his actual fate remains unknown, however, as an article she read says he disappeared).
The whereabouts and activities of Graf Herman remain unclear, even though he survived his battle with Negi and promised that they would meet again. He suggested that he may know how to reverse the petrification cast on the inhabitants of Negi's village when he attacked it some time before the main story begins, but by the "Where Are They Now?" Epilogue, Konoka went ahead and cured them anyway and he vanishes completely.
Negi's mother, Queen Arika, despite being incredible important and surviving the events that destroyed her country, is never mentioned again. Her reasons for abandoning Negi, or her inability to help, for the less cynical, are never revealed.
What Measure Is a Non-Human?: Negi and Team Good fall squarely on the side of "has a mind, is a person". Kurt Godel seems to think only the human inhabitants of the Magical World are worth anything. Since it's been shown that only humans can survive the Lifemaker's "Rewrite" (meaning that the Rubber-Forehead Aliens and Half Human Hybrids might be magical constructs) he has a reason. Not a valid one (they're still sentient, intelligent beings), but a reason nonetheless.
What the Hell, Hero?: Not quite so serious one in chapter 349: Yotsuba calls out the girls for trying to force Negi to say who he likes, if anyone in particular. Her rationale? He's only ten, they already know about his magic and parentage, several of them have pactios with him and he's trying to save the world. Is he really not allowed to have any secrets at all?
When All You Have Is a Hammer: Negi has a tendency to solve his problems with kissing; such as creating a soul for Chachamaru, and unmasking Fake!Asuna with a pactio.
Takane, the girl with the shadow puppets. She's supposed to be at least reasonably competent, but has a tendency to go up against stronger enemies (that is, Negi and friends), get her ass handed to her, and have all her clothes blown off.
Any dragons that appear are likely going to be defeated in a spectacular manner.
Essentially any character that isn't Evangeline, a member of Ala Rubra, Cosmo Entelecheia or Negi is pretty much 99% guaranteed to suffer this at some-point in the story.
In the situation of Negi and Jack Rakan, whenever they break a seeming rule, it is brought to our attention, such as Chisame calling Rakan the man with infinite cheats, the one time he doesn't break a rule.
Wuxia: Euro-centric magic and fantastic setting aside, the manga is increasingly gravitating towards this genre. Ku Fei's, and therefore by extension Negi's fighting style is emblematic of those found in the Classic Shaw Brothers Kung Fu films and Jet Li films of the 1990s.
Xanatos Gambit / Crazy-Prepared: If the fandom's theories are correct, Chao Lingshendefines this trope. Basically, she's trying to avert some disaster in the future. The best way to do this is to reveal magic to the world. But if Negi is able to stop her, then that means he will also be able to avert the future disaster... well done, my dear. Well done.
Year Inside, Hour Outside: Eva's "resort-in-a-bottle" (24:1 ratio), Theodora's similar training ground (10:1), and Eva's scroll (max of 72:1, though it can only be used for mental training, not physical).
Yin-Yang Bomb: Kanka is what you get when you combine the opposites that are Ki and Magical power. It is powerful enough to be considered a power category of its own, but is only used by a select few. Takamichi is the most known user and uses it since he is unable to use magic on its own.
Magia Erebea sort of counts, but would really be better called an 'Everything Bomb'. Which is how it works: Magia Erebea accepts all facets of magic equally, light and dark.
You Are Not Alone: The last two episodes of Negima! (the first anime) revolve around this on multiple levels.
You Can Leave Your Hat On: During the School trip, The magical decoys that Negi's team-members make go haywire and put on a striptease for the class (off-screen). It being an all-girl school, It didn't cause much them much trouble.
You Can't Fight Fate: Both straight and subverted, for the same event. Not to be confused with the character named Fate, although fights against him have been unwinnable more often than not.
Jack Rakan vs Fate played this straight as can be. He stood 0% chance of winning, but was so badass in his refusal to go down quietly into the night that he even made Fate question his motives. And he ultimately subverts it. Even after being rewritten out of reality (twice) he proves that Fate can be fought.
You Gotta Have Blue Hair: Red, blue, dark or light purple,turquoise, green… there are hair colours for every taste in this series.
Younger than They Look: Most of the girls. Lampshaded when two of them try to get discount tickets to a movie, but the ticket saleslady refuses to believe that they're middle school students.
Later, Mana gets a hold of a de-aging candy and is able to buy an elementary student ticket. After having a big laugh about beating the system, she realizes that, with the cost of the candy, she actually lost money overall, making it something of a Pyrrhic victory. Then Kaede shows up in a child's body, but tells Mana that it's due to one of her ninjutsu techniques, so it's free.