This is the name of the phenomenon when a "ghost" or "phantom" key is generated when you press some combinations of keys.
It is caused by the design of the keyboard hardware. It has been tested and confirmed that keyboard clash doesn't occur on an English CPC664 keyboard.
For example, pressing Q,A,P will also generate :.
The problem is much bigger in 2 player games.
It is possible to avoid or minimise minimize the problem by carefully choosing the controls. The problem also occurs when using digital joysticks because these are part of the keyboard matrix.
If you visualize a rectangle in the keyboard matrix above, where 3 of the corners are 3 keys you have pressed. Then the key in the fourth corner will also be pressed this is the "ghost" key.