OK storytime. I have encountered a very similar problem to Pwnemon and Chimpakt.
It started with me accidentally double clicking the Find Battle. After a bit of lag I was thrown into two battles at the same time. I decided to just leave the second battle since I can't focus on two battles at once, and the other was with a good friend bugmaniacbob. After I clicked the x on the battle window for the second battle (I did not click Leave Battle, though I did click Forfeit when the message box appeared). At first in the battle against bmb I could see all of Riolu's moves and had the suggest draw button and leave battle button. But then the message saying that I have lost a battle appeared on my screen, which must have been for the second battle, and then this happened in my battle against bmb:
As you can see, the message for the second battle against Guest 4485 appears at the bottom of the battle screen, and I suddenly lose all options for Riolu except for the Instant Replay button.
After clicking in and out of the battle by viewing the lobby, teambuilder, etc., the program still refuses to allow me to continue the battle with bmb. According to bmb, he is still able to choose his moves, etc. I also still have the ability to chat in the battle chat.
At this point bmb suggests that I refresh the page. I do not click out of the battle tab and click refresh on my browser (I use Opera). When the program reloads, I encounter this:
I am suddenly now seeing the battle from bmb's Pokemon. However, the program still recognizes me and signed me in as Oglemi. I can also chat in the battle as Oglemi. However, I cannot use any moves as bmb, nor can I use any moves as Riolu. I cannot see any of bmb's Pokemon, nor can I see any of Oglemi's Pokemon.
At this point I try refreshing once again, and the same screen appears, only the program logged me out and gives me a Guest # username. I cannot join the chat either, and when I try to change my name to Oglemi by using the top left Change Name button, it doesn't allow me to. I then exit the battle screen by pressing the x.
After waiting for about 20 minutes, I logged onto PS! again, and it now has me signed in as Oglemi.
Quite the glitch if I do say so myself. Twas a fun journey.
EDIT: This glitch occurred at about 1:30 pm Central Time today June 26, there were about 260 people logged into PS! at the time.