Non-specific promiscuous compounds can mislead researchers and waste significant resources. This phenomenon, though well-documented for small molecules, has not been widely explored for the peptide modality. Here, we demonstrate that two purported peptide-based KRas inhibitors, SAH-SOS1A and cyclorasin 9A5, exemplify false-positive molecules – both in terms of their binding affinities and cellular activities. Through multiple gold-standard biophysical techniques, we unambiguously show that both peptides lack specific binding for KRas and instead induce protein unfolding. Although these peptides inhibited cellular proliferation, the activities appeared to be off-target based on counter-screen with KRas-independent cell lines. We further demonstrate that their cellular activities are derived from membrane disruption. Accordingly, we propose that, to de-risk false-positive molecules, orthogonal binding assays and cellular counter-screens are indispensable.