This is one of my favorite theories. I know it's a popular one but who can resist indulging it. Jar Jar is such a fantastic buffoon that it's impossible to see him as a master sith genius...until you think about it.

There's so many instances where we can see evidence of this (as mentioned in the article). The most obvious one is the simplest, no one would ever suspect a klutzy fool of being a powerful sith lord. It's the perfect disguise.

Read the attached article for more explanations