From Rob Baily:

When I go to the Global Permissions -> Edit Permissions button for Groups using IE I get a message saying object expected. It happens whenever the page gets focus and I have the first text box as the active field. Not everyone may get the popup as I have script debugging set up. Instead they may just see an error icon on the page.

My debugger points to this line below.

<input type="textfield" id='groupsToAdd' name='groupsToAdd' value="" size="40" onFocus="javascript:checkEditGroup();return true;" />

I looked and I don't see checkEditGroup defined as a function so that would seem to be the problem.

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Looks like a reference to the javascript method checkEditGroup() was accidentally copied here from the page permissions UI.