Sirius and Sirius B (off to the lower left)
Case in point: last winter I was convinced I had finally, after numerous attempts, seen Sirius B (the white dwarf companion to Sirius) through the eyepiece. It didn't stay in view, dang it, but kind of "popped" in and out - the tiniest imaginable point of light nestled right up against the overpowering glare of Sirius. I even stepped away from the telescope and returned to see whether it was still there. And it was... But alas, when I did some calculations at home later that evening (science ruins everything!), I had to admit that whatever I was seeing could not possibly have been Sirius B - it was on the wrong side! (And yes, I did take into account my telescope's mirror imaged view.) So I sadly had to conclude that I was not only using averted vision to see my mystery object, but averted imagination as well.
So yes, an overactive imagination can be a real problem. It is likely a major component of why Percival Lowell and his contemporaries kept seeing all those canals on Mars. (I myself have seen them twice, despite knowing full well that they do not exist!)
But there may be an equal and opposite error possible.
A City on Mars, as imagined by artist Leslie Carr (1951)
Now I am not pointing these things out to make fun of the illustrator - far from it! But what I am saying is there was a Failure of Imagination when designing the image. The implications of colonizing Mars were not thought through, and there was a (likely) unconscious assumption that the styles and norms of 1950 would not be out of place in 2150 (or whenever this future city was supposed to be built).
Another and far more glaring example of such a failure of imagination is when the news media inevitably announce every new discovery of an Earth-sized exoplanet as a "Second Earth", neglecting the too numerous to count differences (or possible differences) between said exoplanet and our home body. It seems that all that counts in such reporting is the mass of the planet and the distance from its sun. But there are a myriad of other qualifiers that must be met before we can even begin to speak of an "Earth 2", among them being age, atmosphere and water content, plate tectonics, possession of a moon, a rotating iron core (to create a magnetic field), prevalence of asteroids and comets in the system, distance from gas giants (if any), axial tilt and rotation rate... failure to match in even one of these traits means we might as easily be looking at a second Venus as a second Earth.
Do we do the same thing when seeing some Deep Sky Object for the first time? Do we assume without thinking about it that "You've seen one globular cluster, you've seen 'em all!" and fail to see that M13 doesn't look at all like neighboring M92?
So we really ought to approach each new object with a mental clean slate - no preconceptions, no assumptions. These are the bad habits that prevent us from seeing subtle differences that ultimately make the difference between one genus and species and another.