Dewford Footprints
If you walk in front of the gym in dewford your feet don't leave footprints

Grammer Error
If you use a potion on a pokemon with only 1 damage it will say **** recovered by 1 points

Use dig without knowing it
Have a pokemon that knows dig and is really, really close to growing a level learning a new move. Go into a double battle. Have a really slow, but strong pokemon, and kill and opponent while you have the other one use dig/bounce/?fly?/?dive?. Then it will grow a level and learn a new move. Get rid of dig/bounce/fly/surf. Then the next turn it will use dig/ bounce even though it doesn't know it

Sootopolis Gym
Go to the back of the statue that says "This is sootopolis gym" or whatever, and it will say the same thing.

Dewford Gym Glitch
Go behind the statue on the right, and press A on the wall behind it.

MISSINGNO.
Missingno is in the game. He is alot harder to find than shiney. My guess is that everytime you go inside, outside, or you change a Route/Town, it will move to another route, just like Latios/Latias/Raikou/Entei/Suicune. Plus, its rarer to encounter than a shiney, even when you are on the right route with it. Kawaii Pikiachu from the forums said he encountered it a couple times after beating the Level 100 Battle Tower.

Catch Celebi
Its just a glitch, there are two ways it may be. 1. A picture of Celebi may appear instead of the pokemon you really battle. 2. It really is a Celebi. It very rare, even rarer that Missingno.

Fish on peoples head
When ever there are people in the water, right by land, you can fish on them. Or, you can fish, and if the people in the water are moving around, they could move under your rod.

 

Land on Rock in Sky Pillar Go to the second floor of the Sky Pillar (where you catch Rayquaza, you need a Mach Bike). This part is tricky. On the fourth crumbling area (the first one on the right side), stay all the way to the right hand side (right next to the wall). Fall through the last hole there, and you will land on a rock. 

Walk on Ledge

Go to Sootopolis, and go to the Second Highest set of stairs on the right side of the island.  Go onto the first step, and press the left button. There you go!.

Push Boulder onto Ladder Go to Shaol Cave, and get to the Ice Section. Then go back up. You can push the boulder onto the ladder.

Elevator Glitch Go to the battle tower, and beat the first trainer. Now when it asks you if you are ready, say no. Then save the game. When it restarts, most of the time the lady brings you to a different room.

No Wild Battles while Surfing Its Simple, just surf on the areas that you can dive in.

 

Kill without Removing HPs (Ruby Only)

To do this, you have to super kill your opponent in one shot, like a double OHKO, and they faint without losing any HP's.  PigZiggy from Serebiiforums maxed Calm Mind from Kyogre and uses Water Sprout on a low level Sandshrew, and this is what happened

Mispellings

Mawiles Pokedex: (Ruby Only)

Mawiles pokedex reads as follows:

"MAWHILE's huge jaws
are actually steel horns that have been transformed. Its docile looking face
serves to lull its for into letting down its guard. When the least expects it,
MAWHILE chomps it with its gaping jaws."

This might be a random, or not, here's da pic:

 

The mat in the Pokemon League is up 1 space too far.

 

If you have a bike on Select, you press select and then you keep tapping A on surfing water. The surfing music will be before you surf.

Missingno can be found randomly in the game.

 

 

Thanks to kdogneo2 from Serebiiforums:

If you use an item when you're trying to catch Mewtwo or an item close to the time of having caught it. (Optional?)
Walk up to where Mewtwo was standing.
Save your game at the same place or back at the Pokecenter.
turn it back on and open your game file.
And watch the Plot Synopsis (The history reminder thingy) And you can see yourself move into Mewtwo! Mewtwo hasn't been taken out of the history thing. So you can see corners of your sprite behind Mewtwo!