This is on Windows 8.1.1 x64 Pro with IE 11 and on a fast XPS 8700 SE desktop with lots of RAM and SSD drives.
When opening multiple tabs simultaneously in IE 11 with Norton Toolbar activated, some of the tabs will not open. Turning off Norton Toolbar allows all tabs to load and open properly. This problem exists in NIS 2014 as well. This problem is 100% repeatable on my systems.