The following figure (please disregard the dotted lines) shows the net of a ‘normal’ rectangular based frustum. Let $AB = 2a$, $BC = 2b$, $Q’R = 2c$, $R’S = 2d$ and $AP = AP’ = BQ = \ldots = DS’ = 2e$.
The question is “under what condition will the intended frustum CANNOT be formed?” Of course, a non-precise answer is when $2e$ is too long.