Designing for rodents and other creepy crawlies is one of those things I've never seen show up in any project requirement document, yet experience has tought that these kind of real world problems must be considered in any system design.
Ever consider what happens when a Red Ant walks across the high impedance A/D sensors traces on the circuit board? One of those cases that once you see it, the problem is obvious but all of the field reports and remote debugging facilities made no sense at all.
Conformal coating is an obvious solution to this 'bug' problem, but coating is not a panacea for all problems. Like all things in hardware design there are always tradeoffs.
A very common misconception is that Conformal Coating is a Hermetic Seal. It is used a lot in the Coal Mines, and the Electronic Industry in general, to keep the caustic dust, and other contaminants, off circuit boards.
As Conformal Coating is not a hermetic seal, what real happens is the impurities in the water are kept away from the circuit, but the water itself reaches the traces. Since the water is now fairly devoid of contaminates the water acts more like a dielectric insulator. You never notice it in a low impedance digital circuit, but unless debugging is an obsession don't let it get near a RF tuning circuit or a high impedance Wireless Sensor Network circuit.