In scenario where more than 400++ users added to the Project Role users under 'People' tab of a project. JIRA will hangs after you add more users in and popup with either of the below errors:

IE

"Stop running this script?

A script on this page is causing your web browser to run slowly. If it continues to run, your might might become unresponsive."

IE/FF/Chrome/Netscape

"The JIRA server could not be contacted. This may be a temporary glitch or the server may be down."

This is reproducible in a vanilla JIRA instance by using Firefox 16, IE8/9 and Chrome 23.