Contrary to popular belief, the lying friend isn’t automatically an awful person. But she is one with poor communication skills. Usually the woman with the schedule conflict cares about the friendship so much that she’s afraid of sending the wrong message by speaking up about her desire to do X instead of Y. She’s fearful that she’ll be sending the message that the friendship is not important to her. So she lies, gives another last-minute excuse or claims illness or that her mother needs her.
Then days, weeks or months later, the friend she was trying to protect discovers the truth (thanks Twitter, Foursquare, Facebook). Now the friend is doubly offended, and feels disrespected and unable to trust her too. Being honest about intentions early on could have avoided a whole heap of back-end problems.