The sin that plagued the FBI two decades ago, and that now lingers over the Russia case, involves the omission of material facts by agents applying for FISA warrants in sensitive counterterrorism and counterintelligence cases.
Such omissions are a serious matter at the FISC, because it is the one court in America where the accused gets no representation or chance to defend himself. And that means the FBI is obligated to disclose evidence of both guilt and innocence about the target of a FISA warrant.