In Ubuntu 12.04, my mouse clicks when I release the button.
What happens (bad): I right click (mousedown), the context menu appears, when I release the mouse button (mouseup), the item under the cursor is clicked.
What should happen (good): I right click (mousedown), the context menu appears, nothing happens when I release the button. To click an item in the context menu, I click it normally with the right mouse button.
I have experienced this behavior in Chrome, the file browser, and in gnome terminal. The mouse is a Razer DeathAdder (but I'm just running whatever drivers Ubuntu picked automatically), and if it matters, I'm using the AMD/ATI graphics drivers.