I had the same confidence. I put a potential employee through a battery of programming tests, and he passed with flying colors. But when he began work, he wasn't so good. And he was a slob, too (not that I cared).
Four years later, I came upon him in a business suit. I was flabbergasted...never before had he looked even marginally cleaned up (except at that first job interview).
There was a tap on my shoulder. I turned around, and there he was. Again. All sloppy like normal.
I...was...confused.
I looked forward, and there he was in in the suit. And behind me again, there was the familiar slob.
The slob said, "Say hello to my twin brother. And by the way...HE's the one you interviewed...I was the one who took the job. There's no way I could have passed your test."
True story...good enough for me to have enjoyed the quality of the deception more than having resented the depth of the deceit.