Is that quote intentionally saying that you have contradicting premises, and by extension is the statement in that sentence that contradictions do not exist contradicted by the later part of the sentence, or is there something about the sentence that I'm not understanding?
It reads like that at first. What it's saying is for example:
Assume x is 5.
Assume function TimesFive multiplies things by 5.
When we do TimesFive(x) and my answer is 50 instead of the 25. I check the TimesFive function and it's just "print x*5". But how is that possible? 5x5 is 25!
The problem in this case is my assumption that x would be 5 was wrong.
I've had this issue before in my code (though not as trivial an example of course), but it's always a good reminder of something to check when troublshooting.