  We consider a real scalar field with an arbitrary negative bulk mass term in
a general 5D setup, where the extra spatial coordinate is a warped interval of
size $\pi R$. When the 5D field verifies Neumann conditions at the boundaries
of the interval, the setup will always contain at least one tachyonic KK mode.
On the other hand, when the 5D scalar verifies Dirichlet conditions, there is
always a critical (negative) mass $M_{c}^2$ such that the Dirichlet scalar is
stable as long as its (negative) bulk mass $\mu^2$ verifies $M^2_{c}<\mu^2$.
Also, if we fix the bulk mass $\mu^2$ to a sufficiently negative value, there
will always be a critical interval distance $\pi R_c$ such that the setup is
unstable for $R>R_c$. We point out that the best mass (or distance) bound is
obtained for the Dirichlet BC case, which can be interpreted as the
generalization of the Breitenlohner-Freedman (BF) bound applied to a general
compact 5D warped spacetime. In particular, in a slice of $AdS_5$ the critical
mass is $M^2_{c}=-4k^2 -1/R^2$ and the critical interval distance is given by
$1/R_c^2=|\mu^2|-4k^2$, where $k$ is the $AdS_5$ curvature (the 5D flat case
can be obtained in the limit $k\to 0$, whereas the infinite $AdS_5$ result is
recovered in the limit $R\to \infty$).
