This person is saying terrible things about you ... A friend has tagged you in a photo ... I can't believe this is you in this video. You click the message and, hey presto, someone has hacked your twitter, your facebook and who knows what else. You might not even know you've been compromised, unless a kind friend who has received spam from your hijacked account decides to let you know. Do you use the same username and password for internet banking and social media? System meltdown.
Almost as bad is when someone munches your cookies. You go to Goodreads, facebook, twitter, Google, webmail or whatever and hit the first letter of your username. It doesn't fill in the rest like it usually does. You type it in and wait for the long forgotten password to automatically appear as a row of asterixes. It doesn't. This can happen accidentally (using too broad a brush when clearing out internet history so no-one knows you've been looking at cute puppies), deliberately (if you let comeone else use your computer and they trash your cookies while eradicating their forensic trace evidence, because who can you trust these days?) or inadvertently (e.g. when you start using a new browser or your employer does an upgrade that wipes the slate clean or your hardware / software spontaneously combusts because it just knows your life is becoming dependent upon it). But when those cookies are gone, they're gone.