ومنها: أن لا يكون الأوسط متشابهاً في المقدمتين لاختلافه بالقوة والفعل
Another error is that the middle term is not the same in the two premises because of a difference between potentiality and actuality.
In potentiality (bi-l-quwwa) means possessing a capacity whether or not it is currently exercised. In actuality (bi-l-fiʿl) means exercising it at the moment or having the realized state.
Someone may have the capacity to speak while currently being silent. Confusing the capacity with its exercise makes a single word carry two different meanings across an inference.