In Chapter 9 we encounter Moshe bring upon the Egyptians the plague of "Dever" - plague/disease . Moshe approaches Pharoah and warns him that if he refuses to "Let my people go" he will bring about the next in the series of plagues. He goes on to describe it saying the hand of God will smite your cattle, horses, etc., a very serious plague. In 9:4 we see Moshe add a discrimination. he says, "Hashem will distinguish betwene your cattle and the cattle of the Jews and none of the Jews' cattle will die".
Why did he have to add this remark? He never added it by any of the previous plagues, so why now? Anyway, Pharoah probably aready realized that no Jew would be affected, just as they were not affected by any of the previous plagues. So why did Moshe have to mention it this time?
After seeing Pharoah change his mind after each of the previous plagues, Moshe felt it was time to up the ante. Killing animals might not even have been such a big deal in Pharoahs eyes. Sure it is bad, and the financial loss could have been devastating, but thinking of what the Egyptians had already suffered through and were going to suffer through, the animals dropping dead does not seem like such a big deal.
It might well have been easier, at least psychologically, for Pharoah to have gotten through this one than any of the others.
So Moshe "dug it in". He rubbed salt in Pharoah's royal wounds. He told Pharoah that not only are your cattle and animals going to drop dead, but the Jews will not lose even a single head of cattle. That is an infuriating thing to be told. It would have enraged Pharoah. In today's terms, nobody wants to look like a "friar" and that is exactly what Moshe is doing to Pharoah.
That statement of Moshe's might have been even worse than the actual plague.