Messing with people is fun, pure and simple. The only thing that could make that sweet sensation better is to mess with them in a way they don't even notice, at least not right away!
Gesundheit!

“When someone sneezes, I tell them to shut up and they almost always reply ‘thank you’ because they are so used other people saying ‘bless you.'” (Source)
A Little Switcheroo

“I slightly rearrange the stuff on the fridge when I go to people’s houses. Sometimes I leave things where they are, but turn everything 180°.” (Source)
A Possessed Keyboard

“Get an extra wireless USB keyboard at your job and plug the dongle into a victims computer. Then – just a few times per day – type a couple of letters on the keyboard from afar, give them a ctrl alt del, etc. They’ll lose their mind.” (Source)
Great Party Trick

“I have a lazy eye and can make it look forward or at my nose at will without my glasses.
So naturally, I love to meet new people and after a small amount of time clean the glasses while having a conversation.
All the while moving my eye back and forth, watching them try their d—dest not to say anything.” (Source)
Game Of The Thrones

“Say common things just barely wrong to where you know they notice but won’t correct you. Game of the Thrones, Snapchap, Los Vegas.” (Source)
Wait, What?

“Anytime I am saying goodbye, I will genuinely tell them, ‘See you tomorrow!’ It always stops people in their tracks trying to figure out if they forgot something really important or where they would see me.” (Source)
A Magical Fruit

“In the first few weeks of starting a new job, I kept pointing at the basket of bananas in the break room and asking, ‘Hey, I keep seeing people take these. What are they for?’ and then having a coworker explain bananas to me. I’d usually walk away after saying, ‘Oh, I had only read about them in books.'” (Source)
What’s His Name Again?

“Simply introduce myself to a group of people as different names and watch as one forgets my name and asks his friends what my name is to get half a dozen different ones. Works a treat with drunks.” (Source)
Total Recall

“I have a pretty good memory when it comes to stories that my friends have told me, so I like to retell their own story to them a year or two later (now starring myself) and see how long it takes for them to catch on. They start off with a look of, ‘Oh man, I can tell a similar story as soon as he ends his!’ Then it turns into a look of ‘Wait, that’s my f—ing story!'” (Source)
Master Class

“A buddy of mine will randomly wish me happy birthday on Facebook when it’s not my birthday, so then a bunch of other people will wish me a happy birthday and I have to explain it’s not my birthday. This f—er does it every year.
A couple of weeks ago he posted on my wall, ‘Congratulations man, I knew you’d land that opportunity, so proud of you.’ That followed by a couple dozen people asking me what I had accomplished.” (Source)
“Did you know…?”

“Make up a hobby/skill for your friend. Over a period of weeks or months nonchalantly describe your friend’s aptitude at this skill to your common friends or colleagues. If you pick something general enough, your friend will encounter lots of questions, and it might snowball. Examples: ‘Dan is a very good artist. He painted this for me (show them a picture of some art someone else painted).’
‘Did you hear Dan won an award for his artistic skills?’
‘Dan is actually a really good singer. He sung in the state championships last summer.’
‘Did you know Margot fixes watches?’
Bonus points if you pick someone you only vaguely know. Then you can deny starting the rumor and blame someone that knows him even better.” (Source)
On Lockdown

“You know those six button combination door locks? I used to press one every time I walked by my boss’ office. Every time he tried to get in it would take two attempts. Every time I tried to get in it took one, because I knew there was a button pressed. He had me change the lock, twice, and never figured out what was going on.” (Source)
Just A Little Fun

“When I’m on a waiting list at a restaurant, I always put the name down as Parpar.
About half the people kind of hesitate or look at you weird. Play it straight and just spell it out if that happens.
When you’re called over the loudspeaker, it sounds like they’re stuttering. ‘Par-par-party of four.’
You can also do weird/funny names that are just barely believable.” (Source)
On The Tip Of Your Tongue…

“I like to describe a specific movie at length claiming I can’t remember the name, then when they give me the name I go ‘Nah, I don’t think that’s it,’ and suggest a different movie.” (Source)
Always Watching

“I used to have CCTV monitors in my office at a pretty large store, so I would call different departments and hang up right before they picked up the phone. And call them back as soon as they turned around. After a few tries they would stand and wait for the phone to ring again but I’d wait until the exact second they turned around. Nobody ever found out who it was. Easily the best entertainment ever.” (Source)
Now That’s Just Cruel!

“I’m a teacher. I have one chapter test where all of the answers are C. It’s my favorite test. Watching the students slowly look around in fear/confusion.” (Source)
Just Gonna Slide This In Here…

“When going to dinner parties, I sometimes bring a framed picture of myself and place it with the host’s family photos. If anyone asks why my photo is there, I make up a crazy story about how my life was saved by the host and let it spread for a bit.” (Source)
You’re…Welcome?

“In public places, like malls or grocery stores, I nonchalantly thank random people.
I say, ‘Hey, I saw what you did back there. And I just want to say thanks. That was great. Most people don’t appreciate little things like that anymore. But I do.’
Then walk away.” (Source)
Juggling Act

“Get them heavily involved in a good conversation, while standing, and just hand them something. Don’t look at it, keep the conversation going, keep eye contact, and hand them anything from a tennis ball to a rock you just picked up. Almost every time they’ll grab it without thinking. Works even better if talking and walking. See how much stuff they can hold.” (Source)
Betrayed By Good Manners

“Hold the door open for them when they’re still quite a distance away. Usually they’ll break into an awkward shuffle to get there quicker all the while thanking you for it.” (Source)