I keep receipts. Being in San Francisco a week ago, darting and jumping through that city without my usual pens on-hand, I was super-conscious of my receipts because I wasn't updating my checkbook or my little list of credit card purchases. After I got home Monday (finally, though to be honest it was actually very early Tuesday when I got home), I started sorting that jumble of slick paper. I saw one I couldn't account for. A Monday afternoon purchase that I didn't remember, at a time when I was walking from my hotel to the nearest BART station, for an amount that didn't make sense. My response: "...Zuh?"
I wracked my brain for a while, checking my bank account online (great invention, definitely) for the debit to appear, and left a line empty in my checkbook register to enter it once I knew what It was. Yes, I'm anal like that. I'm finally guessing my best guess: I think it was someone else's receipt, made in the Powell Street BART station right before I got there, and I grabbed it and pocketed it (maybe thinking I'd throw it in the trash and just forgetting to do that).
So you're welcome, Anonymous BART Rider Person! Your receipt is safe!