When logging into the same account with the HTML client account already logged in (i.e. the first to log in), the HTML client gives the disconnected message, but it doesn't boot you out to the main menu
(You can interact with tabs and such but you can't click on levels)

When logging into the same account with the HTML client being the second to log in, the error message differs from the Flash client
HTML Client Error: Connection to !! Derron (8 online) closed.
Flash Client Error: Your account was already running on this server. It has been logged out to save your data. Please log in again.

(The other error, the one notifying you that you've been disconnected, is the same between both versions)

this one is a bit more on the subtle side so here's a video and a frame from said video showcasing this




   

What was your favorite thread on the original JV? Maybe you have some fond memories in a particular thread, participated in a fun contest, or liked seeing other people's creativity.

Me personally, I really enjoyed scrolling through Jv comics! The amount of artistry and creativity people had was really amazing.

Just like the Superjump glitch where you can clip into the block above with a mine doesn't work like it does in regular PR2

In regular PR2 you can sometimes get the explosion to clip the player into the water above, allowing them to swim through the block in the centre




Also Mines placed adjacent to water seem to not stun the player in Regular PR2 
while it does stun in HTML PR2

If you have an active Speed Burst, getting another item does not end the Speed Burst like in the original PR2 client.
Instead, you will hold the new item while maintaining the boost from the Speed Burst, and when the Speed Burst ends, the item you are holding will also disappear.

Pressing the tab key doesn't select the elements around the screen.

Edit: Seems like the Stats/character parts selector arrows, add to favourite button and the "?" more info button are the only things I've seen that you can select via the tab key.

After Loading any level from the lobby or the level editor,
backgrounds are then wrongly loaded or positioned

they look correct in the level editor but when you test 
       

Not sure if this actually makes a difference or if it's just an animation thing

HTML:
https://youtu.be/m9qS98SLkd8

Flash:
https://youtu.be/TlgQdYj9k2Y