Contribution gets into another contact's contribution

Sometimes, if there are two contacts opened in two browser tabs and the new contribution page is open in each, a contribution for one contact is added to other and not in the first contact where it was actually entered.
See the video of reproduced error in demo site( civicrm 4.3.4 on drupal, http://drupal.demo.civicrm.org/) : http://www.screencast.com/t/Io6DG9yq2IE8

In the video(Watch FullScreen to see all details): Contribution page is open for two contacts (1)chowski.truman@example and (2) carloss@testing.org as Contributor. I enter contribution for Carloss@testing.org, Status as Completed and submit. The returned message is "No Contributions have been recorded from this contact. QUESTION: Why is no contribution recorded when it was submitted?

Now, see the url of this page. Also remember that Contribution Page for 'chowski.truman@example...' is open. From the same page which was returned on submission I open Record Contribution on a new tab.
Notice that it opens New Contribution page for contact 'chowski.truman@example..." It should have opened 'Carloss@testing.org".

Also the Contribution is recorded for chowski and not for carloss.

This issue is not consistent... it cannot be reproduced always. But it does happen and can be reproduced.