What magic from the HP books is anomalous in some way?

Dedalus Diggle conjuring shooting stars is a big one. The range it implies, being able to cast spells that take effect in the upper atmosphere, is out of step with the range of HP magic we generally see.

Dumbledore casually conjuring hundreds of sleeping bags at once - enough for the entire population of Hogwarts - in Prisoner of Azkaban is another. You can maybe say this isn't an anomaly because it is Dumbledore casting the magic, but even so it is a bit unusual, not least because if he can conjure hundreds of sleeping bags at once then presumably he could also conjure hundreds of lethal projectiles at once.

Another "range" anomaly is Dumbledore banishing Harry's trunk in HBP from Privet Drive directly to The Burrow. Being able to cast a spell in London that takes effect several hundred miles away is a lot longer range than most other magic we see, and also raises a number of amusing scenarios where you could just banish exploding erumpent horns into Malfoy Manor etc.

Any others you can think of?

 

Dumbledore 'conjuring' all of those sleeping bags might be more along the lines of summoning them from elsewhere in Hogwarts. It may not even be him who did it - after all, the elves aren't physically present for feasts but still 'send' the food to the Great Hall, albeit that being a fixed location.

Diggle could be some sort of extremely high-up illusion or enchantment - after all, in Hogwarts we see a night sky in the Great Hall plenty of occasions, and yet the implication is not that the void of 'space' has been summoned into the upper rafters.

'It's in Hogwarts' is a bigger ameliorating factor than it being Dumbledore in many edge cases imho.

No idea about the trunk, that's fucking crazy, the fact that he can do it with an actively magic object doubly so. The Knights of Walpurgis should have woken up to a rain of grenades the night after their first debut.

 

The vanishing cabinets are one I think. There's no implied limit on their range, and they have no downsides. They either can't be warded against, or no one thinks to ward against them. Portkeys seem to be difficult to create, and one use only. Apparition requires concentration, can go disastrously wrong, and seems to get much more difficult when it's side along. It's unknown if there is an actual limit to the distance, but the existence and implied expense of international portkeys implies that for most, it can only be used domestically. Floo travel relies on existing networks, burns a resource, and can be remotely interfered with. Aside from the initial difficulty of making a pair of vanishing cabinets, there seem to be no downsides, and they can be used infinitely.