To the original poster:
I think we should clarify there is no evidence here that Instawallet is acting like Mybitcoin. Please consider updating the title of the thread or adding a note to the first post.
What has been suggested is that attempts to view/use more than one Instawallet at a time can confuse the site, the browser, or both. That should be brought to the attention of the Instawallet operator immediately, especially by someone who can provide an exact example.
(This thread may have enough detail already -- experiments might be hazardous to your balance!)
Many new web services exhibit issues when multiple tabs/windows are opened in the same browser, or when the 'Back' button is used, or when a 'Submit' button in a form is pressed repeatedly. I recently noticed a certain Google service acts erratically or hangs (without any error message) after I open a second browser tab and log into a different Google service with a different user name. If you have dozens of tabs open, it's hard to avoid these situations.
Where irrevocable transactions are concerned, this sort of issue could conceivably lead to a loss of Bitcoins or payment from an unintended wallet.