Everyone lies. It's necessary for us to function in a socially acceptable way. I'm not talking about major deception like cheating on your partner or swindling money. I'm talking about answering "fine" when someone asks you how your day is going. Your day could be crap but most people do not want to know this. We lie to make people feel better, to maintain a social facade and to make ourselves feel better. Small lies of tolerance are what make intimate relationships bearable.
I'm not saying that Pisces is the "liar" sign. But it is the sign that blurs boundaries, that avoids confrontation and is intuitive enough to sense another person's distress. Logical Mercury is in detriment (uncomfortable) in this sign but it's actually a pretty nice combination. Mercury in Pisces = intuitive communication.
Here's an example of someone lacking intuitive communication: Penelope Trunk has Asperger Syndrome (a variation of Autism). High IQ but an inability to read social cues. She is a successful career woman nonetheless and I like her blog. But there are some stellar examples of how blunt honesty is not always best policy. Like when she Tweeted that she was having a miscarriage during a board meeting (she lost 70 Twitter followers over that one). She writes that she doesn't understand why this was inappropriate.
Ok, so this is an extreme example but you get my meaning.