Consider a function field $K(t)$ where $K$ in any field, and define the degree valuation as:
$$v_\infty\left(\frac{f}{g}\right):=\operatorname{deg }(g)-\operatorname{deg }(f)$$
Is $v_\infty$ a complete valuation?
Basically this is the valuation attached to the point at infinity of a smooth projective curve over $K$. For each other finite point $v_x$ of the curve the evaluation is in general not complete, thus we have to switch from $K(t)$ to $K((t))$.