I've never told my parents this, so Mom, Dad, if you're reading, sorry in advance.
When I was really young (maybe 5 or 6), my parents used to make us ask their permission before we ate a piece of our Halloween candy. So, because I was a "good kid," I made a big deal out of asking -- at least whenever mom and dad were around. When they weren't there, I took as much as I wanted.
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I figured they'd eventually notice that candy was disappearing from my bucket more quickly than it should have, so I started eating some of my brother's to compensate. Then I'd dispose of the wrappers behind the basement couch so there wouldn't be any evidence.
Of course, eventually the trash was discovered, and that led to rather lengthy interrogation in which my dad played good cop, bad cop, and "very-disappointed-in-us" cop -- all in an attempt to get either my brother or I to fess up. I lied and professed my innocence, while my brother kept insisting he hadn't done anything (which he hadn't). Subsequently -- "to keep things fair" -- we were both punished.
But, because my brother's candy bucket emptied so much faster than mine (since I'd been eating it), and because I'd been so over-the-top about asking permission to eat my Halloween loot, my parents came down much harder on my brother. In fact, as my dad put us to sleep in our bunk beds that night, he basically apologized for having to punish me in the first place (because, he implied, we all knew my brother was the real culprit).
That was 20 years ago, and I still feel a little guilty. And, now that I'm a parent, I'm sure the bad karma I collected that afternoon will pay me back in spades. At the time, however, I thought it was pretty awesome that I'd gotten away with such an elaborate scheme.
So. What's the biggest lie and/or best excuse you ever told your parents?