I personally really appreciated the alien race in Signs that had managed to travel millions of miles to get here, monitoring our planet for at least part of that journey and possibly before (let's take that planet F'Rahk-kul... No I need to pee, let's stop on the small green one there) but nonetheless never had picked up on the fact that two thirds of it's surface was covered with a material that was lethal to them.
Having typed this I do realise that less than a percent of our planet is actually water, and it is possible an alien race will view reality different from the way we do and they just never expected from their readings that that one percent would all be readily available to the inhabitants of the planet. Statistical likelihood of anyone on earth having water at all is very low, if you figure how few of the molecules around us is are actual watermolecules.
The same goes, to an extent, for the Footfall race, as you cannot plan or prepare for deception if you are incapable of imagining it. Humans will/might eventually be trying to conquer a distant planet and infallible as we imagine ourselves to be, and well planned as the attack will be, we will still be completely rebuffed almost effortlesslyby the inhabitants Sgh-lggl, which we didn't know about, can't explain and have no defense against.
The martians in the newish War of the Worlds, the Tom Cruise version, had been hiding out on our planet for yonks, though, so they were just stupid, which might also be a fair explanation for neglecting a fatal flaw, what with parsimonious thinking and all.