The iTunes 10 update fiasco: Or, in which Apple screws its customers again.
This is not unique to Apple, of course. A number of Microsoft games (Halo 2, for example) require DirectX 10 — as in, the installer will refuse to run unless you have DirectX 10 installed — even though they use no DirectX 10 functionality. DirectX 10, of course, is available only for Windows Vista and later. There is no apparent reason for this other than Microsoft trying to force Windows users to buy upgrades to Vista.