Essentially, there are some that grow so angry at God or the people of God due to happenstance and circumstance and whatnot that they turn from Him completely, to the point that they actively pursue ungodliness with a fiery vengeance, to separate themselves so far from God that they can never return. They become reprobate, they harden their hearts and such to a degree that the reason they don't repent is they arrive at a place where they simply can't. What I mean by this, is that they are so angry, so drawn away, that even if they speak the words, "Please forgive me", the truth doesn't reach their heart and as we all know, Jesus looks upon the heart, not the surface and their main reason for speaking said words is merely in fear of judgement and hellfire, not genuine repentance of their sinful lives.
I believe this is the exception, rather than the rule, to those who belong to Christ. As others have said, we can backslide, slip up, fool around, mess up, etc. and that doesn't put us in this portion of Scripture. However, it is possible, which is why we should always look at our hearts and our lives and our love/commitment to Christ, so as to not blame Him and grow cold, distant and angry with our perfect Savior due to feelings and misunderstandings and the failings of man.