Chapter 7 Exponential and Logarithmic Functions - 7.6 Solve Exponential and Logarithmic Equations - 7.6 Exercises - Skill Practice - Page 519: 2

When do logarithmic equations have extraneous solutions? A: Whenever the argument of the logarithm is less than zero.

The domain of a logarithmic function generally does not include all real numbers and in addition, $log(a)$ is only defined for when $a\gt0$. Thus in any case where the argument (a) is less than zero, the solution is extraneous.