In mouse events jQuery does not translate the mouse button index correctly into the e.which property in Internet Explorer. It works when only one button is pressed at once but it fails when you press multiple buttons.
You can watch the bug in this demo: http://www.ailis.de/~k/permdata/20100427/whichbug/
Press down the left mouse button (Somewhere in the page), then press the right mouse button, then release the mouse left mouse button and then release the right mouse button. Do this in Firefox and you will see that the log output is correct. Do it in IE and you see that the mousedown of the right mouse button is reported as a mousedown of the left mouse button. But the mouseup events are correct in this example. The same can be reproduces with the middle mouse button.