Step 1: You lend an Oyster card to a friend.
Step 2: The friend is an absolute s*it, beyond belief. You're still friends, but not enough to trust them with your Oyster card.
Step 3: You ring Transport-for-London regarding a travel card-Oyster. And they advise you (with persuasion from yourself) to cancel the current Oyster card you have...or... you have "lost"
Step 4: You cancel the Oyster card you know your "acquaintance" is using... and gain £11.00 in the bargain...
Step 5: You innocently text the user of the Oyster card to let them know. Who said shit doesn't happen.