What would be the result of accidentally applying a UHF signal to a VHF power amplifier? Say a 70cm transmission from a dual-band 2 meter / 70 cm radio into a 2 meter PA, assuming the input power is within the PA's specs?
That depends, but generally speaking, your amplifier will just be "too slow" to follow the signal, and you'll just get a mishmash of frequencies at the output. Usually, nothing bad would happen (aside from you emitting at frequencies where that's illegal). Nice thing about having energy at the wrong frequencies is that it stays the same energy, even if somehow converted to something amplifiable by the PA.
It really depends on your device, but some amplifiers might even have an input stage with a moderate low-pass characteristic on purpose – you really don't want to amplify harmonics of the intended signal, anyway, so designing an input stage that has some series resistance followed by a shorting capacity comes pretty easy (or accidentally, actually), and that's only for the better. In this case, your UHF signal will possibly already be canceled when it would reach the main gain stages.
Note that things might look a bit different for very sensitive amplifiers, or in situations where your tone might lead to nonlinearities mixing down energy from formerly out-of-band into your amplifiers operational range.
All in all: Be safe. Have filters. In this particular case, having an RC actually is easy enough :)