Have you ever noticed that when you close a tab in IE using the tab’s red-X button (or middle-click):
…the tab to the left gets wider (or narrower)?
If you then move your mouse, then the tab button size goes back to normal:
This is probably the most commonly reported “bug” that’s not a bug in the IE UI.
So why does IE do this?
The tab button resizes so that if you’re trying to close multiple tabs, you don’t need to move the mouse. You can just click, click, click to close multiple adjacent tabs. The resizing of the tab to the left ensures that the hovered tab’s red-X remains underneath your pointer.
As soon as you move the mouse out of the tab bar, the resized tab returns to its normal size.