There are a couple of challenges. One, memory storage is not infinite -- eventually something needs to be disposed of or overwritten. Two, linkages are not infinite. How is the robot supposed to correlate a current statement or question with a past lie?The problem with lying, they say, is trying to remember the lies -- a robot wouldn't have the problem? A perfect memory?
Artificial intelligence still has a long way to go, and technologists keep lowering the bar on what qualifies as artificial intelligence.