During the early afternoon of 17 October 1952, school superintendent Jean-Yves Prigent and numerous residents of Oloron-Sainte-Marie noticed an elongated cloud-like or cylindrical object moving slowly across a largely clear sky.

This case matters because it is a documented mass event whose object identity remains unresolved.
Documented
During the early afternoon of 17 October 1952, school superintendent Jean-Yves Prigent and numerous residents of Oloron-Sainte-Marie noticed an elongated cloud-like or cylindrical object moving slowly across a largely clear sky.
Debated
The identity of the object
Debated
The adequacy of the conventional explanation
Unverified
Extraterrestrial origin
Unverified
Any claim stronger than the record supports
Sources are kept in separate layers: official and contemporary material is documented; secondary UFO literature is treated as debated, not as primary evidence.
Secondary / debated material.
Secondary / debated material.
Documented source.