1)if someone filled out a lot of MBTI/Socionics tests before so they already have an idea which answer is most likely related to a specific cognitive function
2)some people cannot stay objective /or look at how they acted over the course of their entire life, they rather answer the questionaire depending how they act/think/feel for the past week or month or in the present. For instance: I decided midway in the test that being an infj would be cool too instead of being an intj - so you end up manipulating & messing up everything
3)people can missinterpret questions. I stumbled over a couple ones that could have different implications & left me confused
I share the opinion that personality tests are useful in order for you to reflect upon yourself and get a better picture of who you are. They may work for some people, but if you already did read into cognitive functions and are able to "decipher" the test points/questions, this makes online tests pretty much just a fun way to waste your time away.
(I did the aimtoknow.com test)