Clocks probably have a problem with multiple buttons being pressed because there isn’t a seperate IO line to the CPU for each button.
A computer keyboard is a grid of switches, the D key could (for instance) set of the A, S, D, F, G, etc, and 3, E, D, C keys. Then the processor on the keyboard picks this up, sees the letter in common is a D, and sends ‘D’ to the computer.
The actual layout of the switch isn’t defined, as long as the keyboard sends the right codes to the computer. But if you find a program that’ll show you keypresses, not the letters they generate, and try pressing combinations of keys, likely you’ll find certain combinations don’t work. For me, shift-alt-LeftArrow doesn’t work, but shift-alt-RightArrow does. Was hell playing Doom.
Similarly, alarm clocks with eight buttons (Forward Fast, F. Slow, Reverse Fast, R. Slow, Time Set, Alarm Set, Sleep Timer, Snooze) could either have eight wires directly into the CPU that correspond to these buttons, or it could have three, that being the number of digits you need to represent eight in binary.
(Actually, it would have five, that being the next power of two above 8 * 7, which is the number of combinations of keys, assuming you allow for any two keys at once.)
They might try to get by with less, coding the Fast/Slow Forward/Reverse buttons on one set of two, and the other buttons on the other set of two, allowing for the CPU to distinguish one keypress in each bank of four.
But, the problem comes when you press two keys in the same bank at once. This can result in the CPU thinking a third, different button was pressed, or both being lost, or only the first one being registered.
Anyways, there are reasons why accidentally pressing two buttons they didn’t intend to be pressed together could register as a third different button.