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Circular RNAs (circRNAs) are RNA molecules formed through the backsplicing of linear exons. Several thousand have been identified, yet relatively few are functionally characterized due to challenges in distinguishing effects of circular from linear RNA targets. Recently, CRISPR-Cas13 systems have been utilized to directly target unique junctions formed through backsplicing, potentially allowing for selective degradation of circular isoforms. Applying this approach in pooled screens has indeed identified circRNAs proposed to affect viability in several cancer cell lines. However, the design limitations of applying Cas13d to study circRNAs are not fully characterized. Here, we assessed the limitations of Cas13d-mediated circRNA knockdowns by performing essentiality screens on 900 highly expressed circRNAs in K562, an ENCODE tier 1 cell line. We observed consistent off-target knockdown of linear isoforms by certain circRNA-targeting single-guide RNAs (sgRNAs). Re-analysis of existing Cas13d screens in other cell types revealed similar off-target effects. Using machine learning models that predict Cas13d sgRNA efficacy, we further found that most circRNA-targeting sgRNAs are unlikely to induce strong knockdown. After accounting for these design constraints, 0 of 346 circRNAs testable in our screens had detectable effects on proliferation. Our findings highlight key limitations of junction-targeting strategies, with implications for future circRNA perturbation studies.