Bait and switch. It happens all the time in business and with goods, but what about when it happens in people. You befriend someone based on their apparent personality and then after you get to know them, BAM! their true self shows through with a switcheroo. This happens all the time with acquaintances and "work friends" but the worst two I can think of is when a long-term friend or a significant other pull the bait-and-switch.
You know, that boyfriend, who seems absolutely lovely, and all your friends think he's fabulous....you date him for, oh, 4 months, and all if wonderful, but then, as the months go by, around the 6 month mark, WHAM! he becomes that weirdo you pretend to not see as you walk by on the street. He starts getting irritated with you friends, or ignores you for video games constantly, or releases his inner psycho and leaves you wondering, WHAT THE HECK JUST HAPPENED? You see, what has happened is that he or she has lulled you into that comfort zone, where you feel alright letting out a few of your quirks and then slams you with all the baggage he or she has been carrying around.
Then there is that friend, that girl or guy you knew in high school/college, or lived with in college, that seems like fun and you always have a good time together. Then you graduate and the wild good times end and you have to start waking up at 5:30am and going to a job and can't go out every night. That's when you realize, when the haze of alcohol and cigarettes fades, your friend is not who he or she appeared to be. In fact, said friend, is lazy, and whiny, and only wants to talk about him or herself, only, before it was drunken ramblings and now it is full-time, 100% annoying and you don't know how you got yourself into it, or how to get yourself out of it. It's times like these that you wish there were a lemon law for people.