I have the suspicion that when I lie, I usually have convinced myself at least half way of the truth of my lie. So I cannot really know when I lie.

O yes, I once went to American Apparel and tried to return some t-shirts, still sealed and unpacked, which I had bought several months earlier, and presented the receipts for some identical t-shirts in a different size which I had bought just the other day. The cashier got really confused when she tried to scan the receipt against the t-shirts, and she even called her supervisor. I found that situation quite funny, and explained the discrepancy, but the two ladies became very angry at me, and accused me "why did you lie!" When it was totally obvious why I lied. Just that technology was smarter.