What he did to the company to cause employees to resign is unforgivable.
In this sentence , i bolded the noun clause,
and the entire sentence is considered correct with clear meaning.
He made a mistake to cause the employees to isolate him.
But in this sentence, the entire sentence has the same structure as the noun clause above ,
but the sentence is not considered correct or it is ambiguous,
Why and what causes the difference?
The second sentence seems erroneous.
If the first sentence is what actually happened, then it tells us that what the person did to cause the employees to resign was real evil. In this sentence, the person had a reason to do that; his actions were planned.
The second sentence could be written like this:
He made a mistake which caused the employees to isolate him. If it was indeed a mistake that led to the employees isolating him, then the person had no idea about what he was doing.
The use of "to cause" in both sentences tells us that the action was done intentionally/purposefully. "Mistake" on the other hand happens with no predetermination.
I suppose you might be thinking of the second sentence to mean:
He caused the employees to isolate him, which we now know was a mistake
In that case, the original sentence is ambiguous since it might be misinterpreted to mean:
He made a mistake in order to cause the employees to isolate him.
which, as AIQ already noted, makes no sense whatsoever.
Instead, try the following:
He made the mistake of causing the employees to isolate him
(allowing the employees to/making the employees isolate him would be better still)
