The kids returned from a Children's day festival with a bag of more or less worthless trinkets, among them a lovely KitKat promotional ball mouse. Its fatal handicap is the connector - the last time I had a PS/2 in my machine was a decade ago.
The kids were very sad when they heard the mouse won't work so I had to do something.
Fortunately there was an old USB laser optical mouse lying in my drawer, so I decided to combine the USB electronics with the KitKat case.
I had to move the button switches a bit to fit the KitKat layout.
Were I doing it again, I'd keep the switches part of the KitKat PCB as it'd save me some time I spend adjusting it to match the mouse buttons.
I submitted this to TheHackadayPrize hoping to get some Hackaday stickers :-)