No error, it means every other coefficient in the p.g.f. is negative, which is like saying that certain outcomes have negative probability. In the p.g.f. for the double coin example, the coefficient of the monomial for z^0 is 1/4, meaning that the probability of two coins flipping to both tails is 1/4. If the coefficient were negative there, it doesn't make sense intuitively but that's the "magic" when it comes to these magical half coins.