Passwords. We assume a hard to guess and everchanging password will be hard to crack, but the whole point of machines is that it can be pinpointed with utmost accuracy, and everytime someone tells you to use special phrases in passwords, they're also inadvertently saying "hey thieves, here is what to look out for, happy guessing". They're supposed to be more like speakeasies.
I remember long ago, when I was active as Dabran2 on Neopets, there was a vault with nine dropdown menus that you had to guess the combination to on the moon Kreludor. It was simpler and far more effective. To this day, I couldn't tell you what's on the other side (or I'd have to annihilate you and feed your remains to the turmaculus, assuming you believe I made it to the other side).