The compatibility constraints of your side effects: Beeping
Why does the Welcome screen beep if you hit the space bar after the computer has finished starting up? For compatibility with a side effect of the Windows XP Welcome screen. A beta tester asked us why we removed the beeps that were generated if you started typing at the Welcome screen after the computer finished starting up. Well, we didn’t remove the beeps consciously. That the old Welcome screen beeped was just a side effect of the way the old Welcome screen positioned focus at startup and how it handled unexpected input. When the Welcome screen was rewritten for Windows Vista, focus and input were handled differently, and it so happened that the new way didn’t beep under the initial conditions.
The tester in this case was blind and relied upon the beeping to know when the system was ready. The Welcome screen folks had to go back and take extra care to ensure that if you start the computer and wait for the Welcome screen, then hit the space bar, you get a beep.