For loshon hora, so if you do this unintentionally, it’s an עבירה בשוגג. It would, מסתמא, require teshuva like any other aveira b’shogeg, and once the teshuva is done, a person returns to being a beinoni.
The difference between מחשבות זרות and לשון הרע is that the first is only in a person’s מכשבה, which is not fully under the control of a beinoni, while לשון הרע is much closer to a מעשה than a מחשבה. (Although, it would be an interesting side topic to explore whether לשון הרע is different between דבור is not quite the same as a מעשה גמור for all things).