  It is proven that, under some conditions on $f$, the non-compact flat
billiard $\Omega = \{ (x,y) \in \R_0^{+} \times \R_0^{+};\ 0\le y \le f(x) \}$
has no orbits going {\em directly} to $+\infty$. The relevance of such
sufficient conditions is discussed.
