Use 2+ monitors and 2+ desktops ('Spaces' or whatever they're called). When you're expecting to go from app A to app B, instead you're taken to app C. Or, you're taken to an unexpected window of App B on a different desktop.

Also, the Z-ordering of windows seems really odd. If you have multiple windows open from two apps, and you ⌘+` between them, sometimes all of App A windows are brought to the front and App B windows are hidden. Other times only one App A window is brought up. Meanwhile, if you have App C chilling on a different monitor, it may or may not disappear at random.

Even if it worked as intended, it would be flawed when compared with Alt+Tab on windows.