What characters were used: For the first and second match, i used and the other player used
and for the third match, where the desync happened i used and the other player used in black costume.

What stage was used: I'm gonna put them in order: Battlefield, Final Destination, Battlefield

If you played multiple matches in one lobby or if someone quit your lobby and someone else joined or anything weird like that: Yes i played 3 matches. the desync happened on the third one which automacily disconnected me because of the failsafe or something weird like that, but nobody quited or joined after the first match.

If items were on or hazards were on: No items, but i'm not sure about hazards because i don't know what hazards Battlefield and Final Destination have but they have no hazards right? well then nope. no hazards

Anything else notable you think we should know: We used 2 stocks and no time limit for all of the 3 matches. P1 won the first one, and i won the other 2 matches.
Also, i tried going off SSF2 by clicking on the forums page, and the game din't freeze, but nothing was cycling, not even the timer itself or anyone. maybe that's why someone thought i was somewhere where i wasn't and pretty much attacked in the wrong places! if this is the case, then i must have found the source of it! who am i kidding that's not the only problem but still. too lazy to try it.

Edit: Now i desynced even more
First player was Zelda, i was Sheik
Waiting Room
it was 3 stocks, probably no timer
idk about items but it was a competitive stage so don't think so but it desynced at 3 with a target test which was not displaying a number for targets remaining.
now i tried with Fox vs a Tails on Yoshi's Story on 3 stocks too and it failed. Online mode now fails to work for me now.

1st match, 1st lobby
Battlefield
Mario (me) vs Link
3 stocks, no time limit no hazards no items

Characters used: (me) and (opponent)
Stage played: dream land
Multiple matches in 1 lobby/someone quit your lobby and someone else joined/anything weird like that: joined the lobby and played 1 match in which the desync happened.
Items/Hazards: Hazards on, items off

What characters were used: Fox (me) vs. Fox
What stage was used: Battlefield/Final Destination
If you played multiple matches in one lobby or if someone quit your lobby and someone else joined or anything weird like that: I had desync'd 2 times previously
If items were on or hazards were on: no
Anything else notable you think we should know: sometimes it leads to crashes and others it just kicks me to character select

Stage- Smashville

No items no hazards 4 stocks

p1 Megaman p2 Goku

Desynced from the start

3 stock casuals, Smashville p1 p2 seemed to desync after first KO

What characters were usedMario(me) vs Luffy
What stage was usedBattle field
If you played multiple matches in one lobby or if someone quit your lobby and someone else joined or anything weird like thatNone
If items were on or hazards were onNo
Anything else notable you think we should knowno

What characters were used: Donkey Kong(me), Lloyd
What stage was used: Waiting Room
Multiple matches in 1 lobby/someone quit your lobby and someone else joined/anything weird like that: No, my first time online in SSF2 since v.09b
If items were on or hazards were on: both, but no items or stage hazards appeared
Anything else notable you think we should know: The player started acting weird, SDing, crouching randomly near edges, and doing attacks way off from where i was. Sometimes they would stand idle until near the edge of the stage and then kill themselves.

What characters were used: Pichu, Pikachu

What stage was used: Tower of Salvation.

If you played multiple matches in one lobby or if someone quit your lobby and someone else joined or anything weird like that: Only one match beforehand.

If items were on or hazards were on: Neither were on.

Anything else notable you think we should know: N/A.

What characters were used: (me)
What stage was used: Battlefield
If you played multiple matches in one lobby/someone quit your lobby and someone else joined/anything weird like that: This was after a series of matches with the same person
If items were on or hazards were on: Both were on but there were no stage hazards
Anything else notable you think we should know: the game had ended with having 1 stock left.

What characters were used: + + +
What stage was used: Battlefield & Hylian_Skies
If you played multiple matches in one lobby or if someone quit your lobby and someone else joined or anything weird like that: battlefield first match in lobby. Hylian_Skies was second. No new players.
If items were on or hazards were on: No
Anything else notable you think we should know: I have a feeling it might be due to chibi robo. It's difficult to test on maps that aren't on a platform where players can fall off. 3 players were on browser and 1 was on windows executable. Assume p2p over LAN

Questions: What are the most common scenarios for desyncs to occur?

F3RR0

In response to all the questions

What characters were used: Basically it desync with every character i use, but if you want something more accurate, , , and also, the p2 was using
What stage was used: FINAL DESTINATION
If you played multiple matches in one lobby or if someone quit your lobby and someone else joined or anything weird like that: Yes
If items were on or hazards were on: Idk, i usually didn't create the lobbys
Anything else notable you think we should know: Well, i'm playing in Venezuela, basically the worst WIFI from Latam (even Bolivia have a better wifi i think)

thought about it for a while, now i know for certain. Changing between / causes desync on many connections and/or maps/matchups.

I'm not sure if this has been mentioned, but anytime I play at Whispy Woods with hazards off, there's a desync as soon as he starts blowing (game won't recognize it immediately, but Im99% positive that's the cause)