It is accepted that not all ahadith are authentic. One example is: What did Prophet Muhammad mean by “Seek knowledge even as far as China"? It is sensible, therefore, to reject some ahadith. Some Muslims go so far as to reject all ahadith, e.g.:
Quranism describes any form of Islam that accepts the Quran as revelation, but rejects the religious authority, and or authenticity of the Hadith collections.
This is a controversial stance in Islam, and thus far, I've only encountered scholars advising strongly against hadith rejection (although the Wikipedia page lists a few notable Quranists).
Question: What kinds of sin does hadith rejection lead to (if any)?
Among other criticisms, scholars claim that rejecting hadith can lead to sin:
Whoever... persists in denying and rejecting the hadiths is exposing himself to grave danger, because it is not permissible for a Muslim to reach a conclusion that is not based on proper methodology and without following any guidelines, and criticise other scholars (who disagree with him), otherwise he may fall into sin and error. -- Islam Q&A, article 115125
Islam Q&A asserts (a) those who reject ahadith they believe are sound are committing disbelief, (b) those who reject ahadith they believe are unsound are mistaken.
Those who reject the Sunnah as incontestable proof and accept only the Quran go against the collective consensus of the Muslim community at large, and this consists in extreme misguidance and deviation on their part. -- Islam Web, fatwa 284470
Islam Web questions "how those who reject the Sunnah would worship Allaah; the Quran does not provide a detailed description of or the conditions for the prayer, Zakaah or other religious rituals, for instance."
(What is the ruling regarding Hadith Rejecters? provides other examples.)
It may be that the authors of the above fatawa are primarily thinking of disbelief. However, presumably Quranists don't think of themselves as disbelievers (and no matter who you are, you'll be accused of being a disbeliever by somebody).
I'm trying to pinpoint some specific sins that hadith rejection would lead to---sins that are accepted by scholars as sins, but nevertheless are engaged in by those who reject ahadith because they reject ahadith. Are there any?
I could conceive of arrogance being one such sin, rejecting a major component of Islam despite being accepted by virtually all scholars throughout Islamic history. Although, the logic here goes the other way: arrogance would lead to hadith rejection, not the other way around.