So I was headed toward an exit gate at the train station when a young woman approaching from the opposite direction saw me and sped up so that she could authorise her Octopus card first and pass through, forcing me either to wait or move to another exit.
That's a seriously jerky thing to do. But as my card was already at the gate the scanner read it first and the gate opened, and as I was about to pass the woman strode through as though I weren't there. "What the Hell, Lady?" I asked, but she only gave me a look as though I were the one behaving badly.
Worried that she might have scrambled my card's settings I proceeded through the gate and then visited customer service to ensure I wouldn't have trouble using it the next day, but it was fine.
The woman's card, on the other hand, would not be fine. In fact she would find that wherever she tried to exit, her card wouldn't function, because it hadn't registered her entry. That in turn would mean a visit to the fare adjustment window where she would have to explain her entry point so that the proper fare could be deducted and her card could be reset.
In her attempt to save five seconds she cost herself at least five minutes, which given her attempt to mess with me was well deserved. You know, sometimes schadenfreude tastes good.
But the true lesson here is: don't be a jerk; sooner or later it will come back to bite you in the ass.