Realizing you're on the same flight as someone you told you were in a different place. Shit.
The secret of lying involves not volunteering information where you don't have to, and in providing information which, if it can be verified without your knowledge, is true. The remainder, you fill in with vague or broad statements that can't possibly be construed as false, and only when you absolutely have to provide specifics
and hide the truth do you blatantly lie.
If you do it right, and you're not sabotaged by circumstance, getting caught in a lie (e.g. telling someone you'd be elsewhere and then encountering them later) almost becomes an impossibility.