While there are a lot of strange conspiracy theories out there, perhaps one of the most bizarre conspiracy theories out there is one that the primary promoter of is David Icke: that the leaders of the world (and just about anyone who is famous) are actually shape-shifting aliens.
The aliens are often times described as being humanoid reptilians that are from either another planet or universe (although some claim that they are actually the offspring of alien-human hybrids from thousands of years ago) and have actually been in control for thousands of years, and are using their technology to take on a human form and secretly control the human race.
There are several videos on the internet that claim to show some famous person or politician showing some type of reptilian features for a split second, and the reality is that they do not. Many of these videos allegedly shows a person "revealing" some kind of reptilian features are actually the result of camera angle and light reflection, or even or even natural human bodily actions, such as pupil dilation, and just licking lips with one's tongue. Some of these video are even the result of some special effects put in to the video by someone who wants to prove that shape-shifting aliens are real.
Taking into account the wide scope that many conspiracy theorists believe this to be, many people have been accused of being a shape-shifting alien (this includes David Icke as well, and possible even myself). There are even people who actually claim to be one of these aliens. Of course they never actually turn into an alien, they just act very strange when they are in what they claim to be in their transformation. These "transformations" seem more like acting, or psychosis.
Besides considering the scope of such an alleged conspiracy (in both time and number of people involved) and the sheer lack of any evidence what so ever that any of this is true, it makes no logical sense that any technologically advanced conquerors would even bother to do something like this.
Many skeptics believe that the shape-shifting alien conspiracy theory is actually a modernization and rehashing of the many Jewish conspiracy theories (both of which just so happen to be very similar in their accusations), and have actually accused David Icke of being anti-Semitic and that he uses the reference to shape-shifting reptilians as being a code word for "Jews". While Icke does deny this, he does believe that the book The Protocols of the Learned Elders of Zion (which was proven to be a hoax as far back as 1921) is true, and has apparently been flirting with Holocaust denial.
Regardless of the reasons behind David Icke's allegations (whatever those true reasons may be), many people (both conspiracy theorists and skeptics alike) consider this to be complete nonsense, and have accused Icke of being everything from a scam artist to being outright insane (although some conspiracy theorists have also accused him of being a disinformation agent as well).