Most common, I would expect, is someone witnessing an unarmed robbery, and thinking it was an armed robbery.How many observations of supposed armed robberies happen, say, in one year in the US? I have really no idea, let me use n=1000. How many of those observations are confirmed to be actual armed robberies? Yet again I have no idea, there are many possibilities for errors: a cinema set mistaken for a real robbery, a prank, an hallucination, someone boasting he witnessed a robbery while he didn't etc. etc. etc.
With UFOs, people don't often hallucinate or get pranked: judging by what ends up here (or in the blue book report), people actually see something in the sky that they can't identify; and that observation then turns into a UFO story as people try to make sense of it (which is human nature). It takes more mental effort to make sense of it as a natural terrestrial event, e.g. because you need to learn how your ATFLIR rotates to track a target, to identify aircraft at night, or what planets might be visible brightly.
There is another area where misperceptions occur with deadly regularity, and that's aviation. Aviators have died at night because they mistook lights in the sky for lights on the ground; they have died because they took a prominent line on the ground for a horizon in reduced visibility and lost control of themselves. These people had all heard of these phenomena in flight school. There is a lot of subconscious processing that goes on in the mind that stands between an observation and what a witness thinks they saw. You need hours of actual training in IFC as a pilot to learn to not mis-process these cues, and to instead rely on better cues (like the instruments in the cockpit).
When you think about it, the people most likely to see a UFO are amateur astronomers, yet they are the least likely to report one: and that's because they actually spent hours learning how to identify things in the sky.
The occasional UAP witness will not be able to understand enough of their observation to escape making a false narrative as they try to make sense of it.
Might there be actual extraterrestrial UFOs? Maybe. Will an eyewitness report be reliable enough to prove it? Definitely not.