Why do so many Christians misinterpret the meaning of the unforgivable sin?

Many Christians think that blasphemy against the Holy Spirit means to never repent and to reject the salvation that the Holy Spirit offers and Jesus as the Savior, even though the Bible makes it clear that the unforgivable sin means that if you ever say anything bad about Holy Spirit, which would include a slip of the tongue, then you will go to hell no matter what. The definition of blasphemy is that it is verbal, and there's no reason to assume that the Bible did not take into account the fact that someone can change their mind about what they said.