Musar על סנהדרין 178:13

There are other liars close to the first group in level [of lying] but not exactly like them. Namely, those who lie in their reports and statements. These peoples' profession is not to go around and make up stories and deeds that never were nor will be. But when they come to tell over something, they mix in whatever lies they happen to think of. They habituate in this until it becomes part of their nature. These are the liars whose word is impossible to believe. This is as what our sages, of blessed memory, said: "it is the penalty of a liar, that even when he tells the truth, he is not listened to" (Sanhedrin 89b). For this evil has ingrained itself in their nature such that their words are unable to leave their mouths free of falsehood. It is about this that the prophet grieved and said: "they have taught their tongue to speak lies, they weary themselves to commit iniquity" (Yirmiya 9:4).