The specs that we usually deal with require environmental qualification for a whole host of undesirable situations. However, most of them quote mist tests (i.e., being sprayed with a liquid) rather than waterproof tests (being immersed in a liquid). This means that spraying a liquid (for example, when the Captain tells you the story about that woman in Bankok in 1974, while you are sipping your coffee) is OK
, because it's only a spray and it won't penetrate the covers. On the other hand, if you leave a standing liquid (for example, when the Captain tells you the story of that woman in Bankok and her mother in 1975, making you drop your coffee on the FMS), it may well soak through the covers, into the electronics. At which point either the FMS directs you to Bankok or doesn't let you leave.
Once more, it comes down to economics and risk assessments. The qualification requirements are (obviously) far more stringent and taxing for waterproof equipment than for water resistant equipment. Since the risk of a malfunction and its associated costs are small, the manufacturer (directed by the FAA/CAA/other governing body) only uses the water resistant tests.
Just out of curiosity, were you thinking of coffee with or without sugar and with or without milk? Each variable will have an effect. Sugared coffee, for example, will have a significantly greater effect on a mechanical control or instrument than unsugared.
The definition of a 'Pessimist': an Optimist with experience...