Something I see fairly often in submissions:
The first paragraph or so contains a capitalized word that, in context, could be the name of a person, a place, a device, or something else.
And it's not clear which of those is true for several more paragraphs.
Made-up example opening:
The Gizgorth looked blue.
Or possibly aquamarine. The blue shimmered somewhere between cyan and purple, never quite resolving.
Is that an alien's skin color? A region's foliage? A box's outer surface? Who knows?
Mitzi wasn't sure what had happened to Doozledorf.
She hadn't been gone long. But when she returned, Doozledorf was just gone. Vanished into thin air.
How could that happen? she thought.
Again: person? Dog? City? Hard to tell.
(It can be even more confusing, though probably more likely to be intentional messing-with-the-reader, if the proper noun is an ambiguous real-world one. Substitute "Dallas" for "Doozledorf" in the above; again, could be a person or the city.)
So if you use a proper noun in your opening paragraph, try to give some hints as to at least whether it refers to an animate/sentient being or not.
I'm pretty sure that in almost all cases, this is just a matter of the author not realizing that the reader doesn't already know what the noun or name refers to.
Of course, another common approach is to not use any proper nouns in the opening paragraph, just pronouns:
He wondered where it had gone.
He had been at work all day, and when he got back, it was missing.
This approach has the opposite problem: we know that you're telling us about a person and a thing (good), we even know the person's gender (often handy), but we don't know the person's name or what the thing is that's missing.
This one is sometimes intentional, but unless you really don't want the reader to know the protagonist's name (which is sometimes a valid choice), I recommend not being coy about it.
I think that in general, both of these approaches to openings can be greatly improved by providing specific vivid details that help the reader picture what's going on.
But at the very least, except in some unusual cases, I recommend providing both a name and some indication of what kind of thing that name refers to.
This is mostly a subtopic of the larger issue of an author not realizing that there's stuff that they know about the story that they're assuming the reader knows too, but that they didn't actually put on the page. But that's another topic for another day.