You could get in trouble if your code relied on specific contents of the ROM, for example to use it as deterministic random numbers. Also, installing a Kempston

You could get in trouble if your code relied on specific contents of the ROM, for example to use it as deterministic random numbers. Also, installing a Kempston Joystick hardware interface would mess up the default way to set up interrupts, but a couple of years into the Speccy's life, everyone knew how to do it safely. Those two are the ones I remember.