When I put checkmarks next to messages, and then click the "Delete" button on the toolbar, I often get an "Unresponsive script" error.
It doesn't happen every time, but it does happen often.
At first it happened several times (but not every time) when I searched through a few thousand messages, then checked the few hundred messages that met the search criteria and tried to delete them. Sometimes it deleted a few "100, 200, etc." and then ended with an error before finishing. Sometimes it seemed to stop with the error as soon as I clicked "Delete."
Just now it happened when I checked only one message and tried to delete it. So it doesn't look like the number of messages is necessarily the issue.
I attached a screen shot of the error message in the hope that it will help.