On December 1, 1987, Philip Spencer (pseudonym), a retired policeman, was walking on Ilkley Moor in West Yorkshire when he saw and photographed what appeared to be a small alien creature.

This case matters because it is a documented encounter event whose object identity remains unresolved.
Documented
On December 1, 1987, Philip Spencer (pseudonym), a retired policeman, was walking on Ilkley Moor in West Yorkshire when he saw and photographed what appeared to be a small alien creature.
Debated
The identity of the object
Debated
The adequacy of the conventional explanation
Unverified
Extraterrestrial origin
Unverified
Any claim stronger than the record supports
Unverified
That the photographed figure was an extraterrestrial entity
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.