We may know…
–The pudgy, awkward-looking man who turns out to be a first-rate athlete.
–The dignified, upper-crust fellow who watches WWF on TV.
–The straitlaced librarian who has a wild, active sex life with her husband.
–The big bruiser who gets tears welling up in his eyes when he picks up a tiny kitten.
–The poised man/woman who is an absolute demon behind the wheel. 
Post your own examples of don’t-judge-a-book-by-its-cover people.
My ex-boyfriend, the strong, loyal, man-of-his-word ex-Marine. Turned out to be a complete coward who decided one day to never call again for reasons still unknown. People are not what they seem until they prove it.
How about the friendly, honest, kindly father figure who turned out to be a concienceless, lying, cheating asshole who befriended my SO for the sole purpose of exploiting him and ripping him off.
Three years of “friendship”, when the whole time the guy was spreading rumours and lies, cheating him and backstabbing him. To the point that he appears to have tried to force us into backrupcy, until we caught on to the scheme.
On the more positive side, I know a guy who has a beard, keeps his long hair in a ponytail, wears ripped clothing, has a bunch of tattoos and lots of muscles, and just generally looks like a Hell’s Angel. He is the shyest, sweetest man I know – a teddy bear who wouldn’t hurt a fly.
Just watch a Soap Opera for 5 seconds. Your bound to find the rest of 'em in about that ammount of time 
Yep, I know a guy who looks like a Hell’s Angel. That is, until you notice the collar he now wears because of his high-up position (as a layman) in the Episcopal church. He’s also a very well-respected Ear Nose & Throat specialist, with a great family and awesome kids.
I also have a friend who is a gorgeous, well-dressed, accomplished woman who is forever extending lunch and dinner invitations and befriending people like she’s the warmest soul you ever met. But she’s a psychological mess, low self-esteem, borderline eating disorder, and dishes dirt on every single person she has lunch with. Creepy.
One summer at my former lab, we hired a temp to wash glassware, and uh…I think all the boss wanted him to do was wash glassware.
He was about my age, around 25 at the time. He was a really big guy, well over 6 feet tall and he must have been over 300 pounds. He moved slowly and walked kind of funny too, owing to the fact that he always wore cowboy boots. Oh yeah - he was from Texas, which quickly put into perspective the slow drawl in his speech as well as his fondness for Western shirts, way way way too tight Wranglers, and the gigantic belt buckle.
I remember work being so busy that several days went by before my coworkers and I had time to really meet him and get beyond the work related small talk. But from the start, my friend and I were always rolling eyes, wondering where on earth our boss found this piece of work? We didn’t really expect much from this guy because all he was doing was washing glassware.
So one day things are slow enough and and he starts asking me about the work I was doing and I explain all the hows and whys of all the procedures and processes we were surrounded by. To my startled surprise, he understood every bit. Turns out he’s a degreed MECHANICAL ENGINEER!
::FOOP:: All of my stereotypes and preconceived notions about the backwardness and unsophistication of Southerners and Texans just evaporated in a heartbeat.
He came to Alaska looking for a job and began to run out of money before anything solid came up, and my boss more or less hired him off the street. He was one smart man, probably one of the top ten smartest guys I’ve met in a decade. He taught me a lot of things in the few short weeks he was around. Among other things, how to swallow a lot of pride and do what had to be done to scrape by. Skills I would use myself a year later.
I helped him land a job at an engineering firm two weeks later.