Hard to believe that this could be a design fault. The cars are put through tortuous water tests and with so many 3rd and 4th generation OBs and Legacys out on the road we would have heard a lot more about this. That doesn't mean something isn't wrong, but it might not be an inherent design issue.
Below the windshield there's a black plastic cover. It has some openings and small drain holes, but these are not critical drains. Below the cover is the cowl area, a fairly open space that extends across the car below the windshield. It encloses the windshield wiper motor and the wiper linkages, and an opening for air that's drawn into the cabin by the heater/AC. The cowl has a number of fairly large holes at the left and right sides to drain water that gets in. The plastic cover isn't a tight fit, and water from its openings drains into the cowl area. The black cover's holes are generally small enough to prevent leaves etc from getting in below, at least anything large enough to block the cowl's main drain holes. However, accumulated small bits could build up and eventually slow the flow through the cowl drains, and if the cowl area were to fill up (this would take a very heavy rain), it could overflow into the heater intake. (The intake is raised above the bottom of the cowl to prevent this, at least in normal situations.) Water would then pass through the cabin air filter to the fan squirrel cage, and then down the duct to a joint where it would drip out onto the carpet. But I think this is a very unlikely scenario.
In any event, the cowl cover can be removed by first removing the wiper arms and then the plastic clips that hold the cover in place. The cowl area is then exposed, and can be inspected for drain blockage, or some other unusual anomaly.
a pine needle can clog the hole and then some evaporator thing fills up and drains into the passenger side floor.
This is not an external to internal leak, but it's also not uncommon. (There's a few threads here relating to this.)
The evaporator is what cools the cabin air when the AC is on. As the air, pushed by the fan, passes over the evaporator and cools, water in the air condenses (changes from a gas to a liquid), runs down the evaporator, and collects in a pan area below. The pan has an outlet to a rubber pipe that goes out the front right area of the firewall. When the AC is on, especially on hot, humid days, it's normal to see water dripping from this drain. (In fact, it's one sign that the AC is working!) But the small hose can become blocked, especially nearer the lower end. ( It can happen overnight if an insect crawls into it and builds a nest.) When it does, water can't drain and eventually overflows the evaporator pan and drips out of the heater/AC duct, which is above the front passenger foot well, soaking that carpet.
The AC is on anytime the heater/AC blower is used with the system mode switch set to one of the two windshield defroster settings; in other words it can be running even if the AC itself hasn't been manually turned on.
So while it's possible there's an external source of the water, I wouldn't discount the possibility that the evaporator drain hose is blocked, and the water on the floor could be from a recent drive with the heater/AC fan on.
All to say that I think there are some things to check out before concluding that there's an inherent problem.