This one is fairly simple: “I cannot understand or believe that something is true, therefore it is not true.” Pretty obvious why this one is fallacious, in that the truth value of a claim is based purely on the credulity of a single person’s mind. It’s easy enough to pick on the Growing Earth people, whose entire “new science” seems to stem from the fact that they think a Pangaea world would look stupid. Creationism, another easy target, fails in that one of their central arguments is the idea of irreducible complexity: “Can you even imagine how an eye could evolve? I mean, please.” Virtually everyone I think, though, commits this one since we humans become so defensive about our belief structures that we instinctively lash out at challenges to those structures.
I think a large part of science education, or at least in my case, is about learning that the imagination of a single human mind cannot remotely fathom the wonder and grandeur of the physical universe, and a fair bit of training must be undergone in order to not reject things like quantum mechanics out of hand for being too weird. Even the greats fell victim to this, like Einstein rejecting the idea of an expanding universe or action at a distance. The way to combat this fallacy is the root of all scientific skepticism: Be open-minded, but demand evidence. If the evidence says something (and was reliably gathered, interpreted, and reviewed), that’s what you have to go with regardless of your preconceived notions.