Many young earth creationists argue that the world's population today can only be explained if there were 6 people about 4400 years ago (as per the flood story of the Bible.) To prove this they assume that the world population doubles each 150 years, and count backwards using that number.
This, of course, makes two major mistakes. Firstly, the number 150 is completely spurious and based on absolutely nothing. It's pulled from one's behind. It has been carefully chosen to give the right answer when you plug it into the equation. So rather than taking a known fact (in this case the alleged doubling of population each 150 years) what they have done is to take the equation and ask "which growth rate would give me 6 people 4400 years ago?" and come up with the answer "population doubles each 150 years."
Secondly, it just assumes that population growth has been the same for the entire history of humanity. That's of course, completely unjustified and can be easily demonstrated as false.
However, the most ironic thing with this is that they do not realize what their growth rate assumptions imply. Just ask them to calculate, using that exact formula of theirs, how many people there were during important events described in the Old Testament, such as Israel's escape from Egypt.
(The story talks about hundreds of thousands of Israelites alone, yet the formula that these creationists use would give just a few hundreds as the entire population of Earth.)