I think that many physics crackpots honestly believe that they've made some great discovery.
What happens next is another story. If they discover that this discovery is rejected by the mainstream scientific community, they have two main possible actions.
Decide that they are mistaken. We don't learn much about that.
Decide that it is contrary mainstream theories that are mistaken. That can lead to a career in crackpottery. They often decide that the reason that mainstream scientists reject their wonderful theories is because those scientists are closed-minded orthodox oxen.
As to physics crackpottery, Martin Gardner had a nice selection of it in
Fads and Fallacies, though it's over half a century old. It's hard to compete with George Francis Gillette's zaniness, though Archimedes Plutonium and the Time Cube gentleman seem like they could. GFG in action:
Each ultimote is simultaneously an integral part of zillions of otherplane units and only thus is its infinite allplane velocity and energy subdivided into zillions of finite planar quotas of velocity and energy.
In all the cosmos there is naught but straight-flying bumping, caroming and again straight flying. Phenomena are but lumps, jumps, and bumps. A mass unit's career is but lumping, jumping, bumping, rejumping, rebumping, and finally unlumping.
His "backscrewing theory of gravity": "Gravitation is the kicked back nut of the screwing bolt of radiation.", "Gravitation and backscrewing are synonymous. All mass units are solar systems ... of interscrewed subunits.", "Gravitation is naught but that reaction in the form of subplanar solar systems screwing through higher plane masses."
Not surprisingly, he complained about what "orthodox oxen" mainstream physicists were for not accepting his wonderful theories.
Martin Gardner also noticed two phases of physics crackpottery:
1. Anti-Newton
2. Anti-Einstein with defenses of Newtonian physics
What might a third phase be? "Back to Einstein"?