4-Hydroxybenzaldehyde is treated with exactly one equivalent of $\mathrm{CH}{}_3\mathrm{MgBr}$ in dry ether and then worked up with dilute $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The organic product (apart from $\mathrm{CH}{}_4$ evolved) is

The phenolic O-H is far more acidic than any C-H, so the first (and only) equivalent of $\mathrm{CH}{}_3\mathrm{MgBr}$ is consumed by deprotonation, giving $\mathrm{CH}{}_4$ and the magnesium phenoxide. No Grignard is left to attack the aldehyde, and acid work-up regenerates the starting phenol-aldehyde. Addition to C=O would need a second equivalent.