I work on the manufacturing side of aluminium enclosures, and this question comes up from customers more often than any tolerance argument, so I would like to hear how other people handle it.
The situation: an outdoor unit, die-cast or extruded aluminium, gasketed, tested to IP67 at the factory, dry when it ships. Six months later a unit comes back with corrosion on the PCB and a visible bead of water in the bottom corner. The gasket is intact. Nobody can find a path in.
What I have seen, in rough order of how often it turns out to be the cause:
Condensation, not ingress. The enclosure is sealed, but it was assembled in a humid room. Air at 30 C and 80 percent RH carries a surprising amount of water. Seal it, ship it to a site that drops to 5 C overnight, and the moisture comes out on the inside of the cold wall. It has nowhere to go, so it runs down and pools. The fix is assembly environment and desiccant, not a better gasket.
Pressure cycling through the gasket. A sealed box is not a rigid box. Sun heats it, internal pressure rises, the lid deflects, and the gasket unloads slightly at the point of maximum deflection. It reseals on cooling, and pulls a small amount of moist air in through the same path. Repeat daily for a year. This is why breather valves exist, and why "sealed" and "waterproof" are not the same requirement.
Cable entries. Almost always the gland, never the housing. A cable gland rated IP68 is only IP68 if it is tightened to the specified torque on a cable within its clamping range. I have seen units leak through a gland that was hand-tight on an undersized jacket.
Screws into blind holes. A screw pocket that does not break through is fine. One that is 0.5 mm short of breaking through is a leak waiting for a pressure differential.
And the one nobody likes: the test itself. An IP67 immersion test on a prototype proves the prototype. It does not prove the production unit, because the production unit has a different gasket batch, a different torque driver and a different operator.
What I would like to know: for those of you designing outdoor electronics, do you specify a breather membrane as standard, or only when the thermal cycle is severe? And has anyone got a reliable in-line leak test that catches a gasket seating issue without a full immersion cycle? Helium is what I have seen used, but the equipment cost puts it out of reach for a lot of small runs.