In various models and systems involving the escape of periodically forced particle from the potential well, a common pattern is observed. Namely, the minimal forcing amplitude required for the escape exhibits sharp minimum for the excitation frequency below the natural frequency of small oscillations in the well... The paper explains this regularity by exploring the transient escape dynamics in simple benchmark potential wells. In the truncated parabolic well, in absence of the damping the minimal forcing amplitude obviously tends to zero for the natural excitation frequency. Addition of weak symmetric softening nonlinearity to the truncated parabolic well leads to the nonzero forcing minimum below the natural frequency. We explicitly compute this shift in the principal approximation by considering the slow-flow dynamics in conditions of the principal 1:1 resonance. Essentially nonlinear model, analyzed with the help of transformation to action-angle variables, demonstrates very similar qualitative features of the transient escape dynamics. (read more)