What practical pitfalls should I expect in my first mammalian CRISPR knockout?

[VictoriaR]

I'm a graduate student in molecular biology, and for my thesis project, I'm planning to use CRISPR gene editing to knock out a specific gene in a mammalian cell line to study its function in a metabolic pathway. While I'm familiar with the theory, this will be my first hands-on attempt at designing and executing a full CRISPR workflow from guide RNA design to validation. For researchers with practical experience, what were the most critical pitfalls you encountered in your early CRISPR experiments, particularly regarding off-target effects and ensuring efficient delivery into your specific cell type? What validation methods, beyond just sequencing the edit site, did you find most reliable to confirm a successful knockout and rule out clonal selection artifacts?