By makas bechoros, Moshe was careful to say that the firstborns will die at approximately chatzos. He didn’t tell them that it would happen exactly at chatzos (as indeed occurred) as Moshe was concerned that the Egyptian clocks might be slightly off, and when they think that it is midnight, and makos bechoros didn’t yet happen, they would say that Moshe is a liar.

The Brisker Rav zt’l wondered, how could they think Moshe was lying? They had already witnessed nine makos! Everything Moshe said occurred. How could they doubt him? The answer is, if one doesn’t want to believe, he can see miracles and proofs before his eyes, and he will still find excuses to deny them, and to attribute it all to nature. ~ R’ Elimelech Biderman Shlita