I was looking at the questions where Everything Joe says is true and I thought it would be interesting to explore the inverse superpower.
Bob isn't just a pathological liar, but if he says a true statement, the fabric of reality will be altered to make his statement false in some way. This takes the path of least resistance and will alter as little as possible to make his statements observably false, so for instance if he states that you scored 99/100 on a test (and you did), your score would suddenly become 98 or 100. If Bob states a paradox, nothing happens. People automatically disbelieve any statements he makes. In addition, this superpower is permanent and cannot be removed through anything he might say or any interaction with other superpowers that might exist. As an additional limitation, any statement that, if false, would harm himself or another human (or bring the end of humanity as we know it) is nullified. He also cannot affect his own memory, personality, or desires through a falsified statement.
There are ways to manipulate this in his favor, such as stating the opposite of what he wants in the simplest way possible, but because as little as possible is changed, this can be tricky in some cases since only one detail of his statement must be false in order to render the entire statement technically false.
The problem is that it's hard to get anyone to trust you when you have a track record of only saying lies and nobody ever believes you. How does Bob build any sort of meaningful relationship of trust with anyone?
EDIT: This effect extends to written language and sign language, so pretending to be mute is not an option for getting around it. It is also affected only by what is literally said, so stating something sarcastically produces the same effect as if it were said seriously. Questions and imperative sentences also have no effect since they do not assert any factual information.