(1) Forgetting the base case: "all horses are the same color" paradox fails at n=1→2 step. (2) Circular reasoning: using P(k+1) to prove P(k+1). (3) Wrong base case: if statement is for n>=2, base case must be n=2, not n=1. (4) Not using the inductive hypothesis: the proof must explicitly use the assumption that P(k) is true. (5) In JEE MCQs, sometimes the question asks which step fails for a FALSE statement — understand that both steps must work for a TRUE statement.