  The idealization of monochromatic plane waves leads to considerable
simplifications in the analysis of electromagnetic systems. However, for active
systems this idealization may be dangerous due to the presence of growing
waves. Here we consider a gainy slab, and use a realistic incident beam, which
is both causal and has finite width. This clarifies some apparent paradoxes
arising from earlier analyses of this setup. In general it turns out to be
necessary to involve complex frequencies $\omega$ and/or complex transversal
wavenumbers $k_x$. Simultaneously real $\omega$ and $k_x$ cannot describe
amplified waves in a slab which is infinite in the transversal direction. We
also show that the only possibility to have an absolute instability for a
finite width beam, is if a normally incident plane wave would experience an
instability.
