My Elanon campaign just reached 6th level and Friday night, the party had just finished their hardest fight.   Most doors in the dungeon had been ornate with unique features; the door from this large room was rather plain.  Except, it opened in and had three iron spikes driven between the bottom and the floor.  The party that had foiled traps, lots of undead,  and other challenges spent 10 minutes arguing if they should open the door.  "There's something really bad on the otherside..."   Leomund's tiny hut, long rest, lots of prep, and they finally decided to remove the spikes, and slowly opened the door...

Inside was the unguarded treasure room...

And a DM with a sore jaw from clenching teeth to not laugh!!!!!! 

My goal as a DM is to tell a story that is enjoyable and memorable.   I think this counts!

That look of frustration and joy on their faces means you are doing your job right!

I'm going to make this one of those "Two kinds of people" posts by saying where I thought you were going with that:

     An unguarded treasure room, where the floor of the entire room is deadly spikes or some other hazard, but you can't see it because the entire floor is covered in gold.  They step in, and right down like a ball-pit.  Kek.

Not sure if this counts as "evil" or really a trick but one thing I've brought as a player to Games I run, but rolling implies a chance at failure so there are times I make players roll, for no reason at all, knowing they succeed but I want to bring out the stress of struggle. I knew the paladin would over power the spirit in sword, that's how it all ends, but let him roll two or three times to emphasize the struggle.

There are ton of tricks I use to keep the party on their toes.  I call them mook encounters.

Mook 1) You spy a wolf on a hill in the distance watching you.  The wolf is just a wolf and has nothing to do with anything.  It may follow for a bit and disappear.  You would be surprised how much time the players wasted on a typical woodland creature just looking for a rabbit to eat.   I've done the same with a Owl flying through a camp at night.  Nothing special about it but because I mentioned it, it must be important LOL!

Mook 2) The players encounter two merchants fighting over space to place their carts of goods.  It's a who arrived first thing.  Some players keep walking and some stop to try to settle the dispute. This can be an awesome roleplay event.  

Mook 3) A child ask for help getting his/her cat out of a tree.  The child might have important info.... or not and just wants their cat back LOL!

Mook 4) Potion labeled healing that actually contains a minor poison.  Nothing that will kill a player but might give them the runs later on. Disadvantage on some rolls. They don't know till it happens and are not sure why the potion didn't work.  Never trust what the bottle might say and always check the EXPIRE Date.

Mook 5) The wife of one of the local nobles suddenly falls in love with one of the characters.  Jealous husbands are awesome! 

I have lots more but don't want to give all the stuff away in case one of my players is reading this and going Damn I hate that guy!

My "evil" idea is similar to Webwraiths: I just make them roll checks randomly while traveling or exploring. I also mention certain townspeople or animals in the wilderness.

For example: "You go to make camp, clear away the brush and build up a fire. As soon as the fire is lit, you notice a dark raven sitting in the tree above. Watching you. Upon your noticing him, the raven takes to the sky soundlessly."

There's nothing special about that raven but it leaves them wondering.

The exact opposite can be humorous, I remember a giant boulder trap and each of us got hit eventually going back and forth.  The DM tells us either acrobatics to dodge or athletics to stop it, which I believe is why we all tried to stop it at times even multiples of us thinking this was part of it.  The source material actually said there was no way short of desintigration the pc's should be able to stop the boulder...

I had this one player, nearly every time he rolled a perception check, he rolled a Nat 20. Going with bonuses for 20 and utter fail on 1, I decided that every time he did that, he seen a hawk with a fifteen foot wingspan, hover high in the air, it had ten colored bands on its wings. With my tenacity for twists and storytelling, they couldn't tell if it was something random or if it had deeper meaning. I didn't actually have plans for it, but it was always watching.

I haven't had the chance to do it yet, but what would be a really funny RP session would be using a really hostile beggar against the party. The beggar could just go on and on and on and on, borderline forceful, trying to get a good amount of gold from the party. If the situation turns south (beggar being hurt, etc), maybe a few townspeople see the event unfold. If they cave in and give money to the beggar, maybe they see him getting shwasty in the local tavern with the money they just gave him. Either way, funny moral RP event that will lead to a good story later down the road.

I always pull the gazebo card and see if anyone tries to attack it.

I do a lot of stuff in the OOC thread of my pbp games. Like Come on - open the door. I promise it's not trapped ....... *chuckles* or I promise the chest isn't a mimic. I'd never lie to you guys. Ever. Really ..... ok it's a mimic. But it has treaaaasuuuuuure. Or ... well, or not, maybe. You'll have to open it to find out. I mean fight it.

Stuff like that. I feel like I'm being hilarious, while being fully aware I'm not. But I'm having fun, so that counts. Right? =)

I do this, btw, because sometimes they ask, or sometimes they guess, and sometimes they're right, but then other times they're wrong. And I feel I have to respond to everything, but the only way I can reply without letting the cat out of the bag is by ... you know, gaslighting. 

One of the best ones has to be the purposefully vague descriptions:

"You enter what appears to be a room. Ahead of you, on what seems to be a stone dias, is what looks like a wooden chest".

"You don't see any traps".

That sort of thing.

I had the party being pranked by a mimic bladderpus (think mimic octopus, but large and able to float) who disguised himself as the dragon they had stolen from, chasing them down. The third time, it wasn't the bladderpus!

I also hand out minor magic items without adequate description. The barbarian found a stick in a compos heap, which "rang with a B-flat tone when struck". He kept it as the "B-Flat Stick". about 20 sessions later, they met an artificer, who identified it as a chime of opening!