What was observed: As Oumuamua moved away from the Sun, it exhibited a slight non-gravitational acceleration, a small push that wasn't caused by gravity. For comets, this is normal; the push comes from jets of gas and dust erupting from the surface as it's heated (outgassing), which creates a visible coma and tail.

Why it's the weirdest: 'Oumuamua had no detectable coma or tail. This created a major paradox: it was accelerating like a comet but looked like an inert asteroid. This forced scientists to propose novel explanations, such as outgassing of invisible hydrogen that was trapped in water ice, or that it was a unique nitrogen iceberg fragment from an exo-Pluto. The sheer difficulty in explaining the visible evidence (or lack thereof) makes this the top anomaly.

***

(2) Oumuamua's Extreme Shape

Based on its dramatic changes in brightness as it tumbled, scientists inferred an extreme shape unlike any known solar system object.

What was observed: The object's brightness varied by a factor of ten, suggesting it was either extremely elongated like a cigar or flattened like a pancake, with a length-to-width ratio of up to 10-to-1. Later analysis leaned towards a pancake or disk-like shape.

Why it's weird: While weirdly shaped objects exist, 'Oumuamua's proportions were unprecedented. This unusual form is hard to create through typical cosmic processes.

Explanations include it being a fragment of a larger body torn apart by a star, which could stretch it out. The extreme shape, while a major oddity, is ranked second because it's a structural anomaly, whereas the acceleration was a behavioral paradox.

***

(3) 3I/ATLAS's Combination of Anomalies

Unlike Oumuamua's single defining mystery, 3I/ATLAS is weird due to a collection of several unusual, "off-kilter" behaviors that, taken together, paint a strange picture.

What was observed:

Unusual Composition: It showed high levels of carbon dioxide and a much higher nickel-to-iron ratio than typical comets.

Lack of Tail (Initially): For its size and non-gravitational acceleration, it was expected to have a massive tail, which was not initially apparent in post-perihelion images.

Weird Brightening & Color: It brightened faster than any known comet near the sun and shifted color from red to green to blue, which is not typical cometary behavior.

"Anti-Tail": At one point, it displayed a jet of dust pointing toward the sun, which is not a normal tail orientation and suggested very slow-moving dust particles being pushed off the sun-facing side.

Why it's weird: While some of these traits could be individually explained (e.g., its composition is a clue to its origin), the sheer number of oddities makes it a profound mystery. It behaves like a comet, but not like any comet we know. It is less weird than 'Oumuamua's core paradoxes because it still displays cometary activity, just in a very strange manner.

(http://www.autoadmit.com/thread.php?thread_id=5795916&forum_id=2/en-en/#49418422)