Lying is important. In Planescape, it was common that two or three of the dialogue options one had was marked with [Lie]. You could lie about many things, not simply quest things like "Did Leandro steal my things?" but lying about personal beliefs, intentions, etc.
I've seen other RPGs incorporate lying, but only P:T would have the same identical lines available, one marked with [Lie] and the other without.
I also understand it would be hard work to incorporate it into the game world, but the ability to spread false rumours, create arbitrary conflicts, and in general get factions, organizations and people into hostilities with each other would simply be... terrific. I don't think I've seen this in any game before, but it was an immensely important tool in the noble's power struggles, historically. To depict a power struggle without giving room to lies is a mistake.
When is it appropriate for lying to require a successful die roll? Discussion and answers to poll appreciated.
Does anybody have examples of when lies have been overused, or badly used, in game before? I'm sure there must be some examples to heed.