I've been experiencing extremely strange and GAME-BREAKING online mode glitches. I don't know the causes, but occasionally when I enter a game of online mode 1 of 3 things happens.
1. The game starts on the correct map but Sandbag is my opponent and we both have 300%. After the game ends, I get stuck on the choose-a-character screen. Every character has the nickname pad up, no buttons will work, and a lot of text and visuals are missing.
2. The opponent and I spawn on the training stage instead of the selected map. It lags really really bad.
3. A combination of 1 and 2.

For 1 and 3, my save data becomes corrupted and I must erase it. It's really ticking me off.

EDIT: Just experienced another one. The game is normal except when it ends, the game says it can't register finished status or something like that. The game gets stuck on a Please wait... screen and I must restart. It also corrupts my save data.

If you button mash when using Super Sonic's down aerial, he stays in place.

When playing on heavy gravity mode, sometimes when you jump and fall very fast to the ground, your landing lag animation doesn't play and you're stuck in your falling animation.

In this state, you can perform air moves on the ground and it still won't correct your falling state.
Also, having dramatic mode on with 3 Jigglypuffs on the stage can cause the game to lag at times where the CPU's, items such as the Smash Ball, different effects such as the smoke from when you do your initial dash are frozen in place, and it can take a while for the game to un-freeze itself.

This has always been a problem with heavy gravity, and Jigglypuff.
Reply to me if this has already been confirmed.

When u play as link on namek and put 999 of damage to CPU and put it to be human,the camera freezes

This was already mentioned by dihaigal when you have the CPU as Jigglypuff and set it on human after it being on idle mode.

[linux] [1981] Linux build's online mode will forever stay on the first "Please Wait..." (when logging in)
[linux] [1981] Keyboard does not work in fullscreen.

Megaman's glue bombs (Crash bomber) attach through invincibility frames :<

Fox, Pikachu and Black Mage receive no damage when Link sends a bomb and crouch directly.

When I first came to this site I was able to play this game just fine and now when i try to start the game this is showing i do not know what to do about it?

Maybe just go to http://forums.mcleodgaming.com/viewtopic.php?f=4&t=40447 and download the game?

I have the same issue as KaotiikLynx only it is with the downloaded version of the game.

Please help I wanna play the game!! D:

http://forums.mcleodgaming.com/viewtopic.php?f=54&t=39352
read this article and follow every step. this should probably fix your issue

https://www.youtube.com/watch?v=VG4b3aGx6h4&list=UUw7JjyIMDwdzsJUfUoH-7ig
Yoshi gets stuck in his Ground pound animation, here's how we did it, On a thin surface (or a platform you can pass through)
Have yoshi use his down special so that he lands on it, and at the same time, hit wario out of his d-air (also a ground pound)
If you really care: this was our reaction (i was playing with a friend)