Bekhorot 30: three things to remember

Rava banned the butcher from selling walnuts because he used walnuts to bribe children into stealing forbidden kidney fat.
Suspects in Torah laws like tithes are untrusted for rabbinic purity, but suspicion does not transfer between tithes and Shemitta.
A prospective convert, chaver, or priest who refuses to accept even a single rabbinic or biblical detail is wholly rejected.