"Lies"
The hardest thing to do in the world is to frame a perfect, internally consistent lie*.
On the surface it looks to be harmless enough to tell someone a small "white lie". But if (or more likely, 'when') questioned about it, you have to make up another lie to cover up your original lie. One lie will lead to another and eventually you'll be lying buried in a pit of your own lies.
Now the truth, by definition, is internally consistent. You may forget the truth, but you don't have to make it up or remember different truths for the same event. That's another problem with lying -- you don't remember whom you've told what version of your lie. (Maybe I should invent a version control system for lies -- I could call it ClierCase**!). With a lie, it becomes tougher and tougher to maintain internal consistency the longer it goes on.
Therefore, the challenge is to create a lie that is completely and totally consistent and will stand up to any amount of scrutiny (and is not 'cake').
Protip: It helps if you convince yourself of the truthiness of the lie.
* Of course, the whole theory is bound by the HUTD, so YMMV (Your Mileage May Vary).
** This counts as prior art for any future patent applications from person(s) (other than me) for any form of version tracking or controlling system for lies.