Surprise, all small keyboards have a hardware bug when using the fn key.

So far no one has discovered it since the fn key was only used together with the F-Keys.
Thanks to the new modifier mapping feature we know the bug exists.

Apple, do you hear us? Fix your key multiplexer! This bug exists in any small Apple keyboard since 2007.

List of key presses which do not work

  1. fn + left control + 5
  2. fn + left shift + T
  3. fn + left option + G
  4. fn + left command + B, pressing B twice drops+reconnects Bluetooth on the older keyboards.
  5. fn + right option + 5, T, G, B
  6. fn + caps lock + 5, T, G, B

If any of these combinations are pressed the keyboard raw data reports 6 overrun errors. Pressing an additional key doesn't help either.

Cases 1. - 4. have been fixed with version
Cases 5.+ 6. cannot be fixed as there is no way to determine which key was pressed.