KKoran, not always so, my aircraft, {an eight pax presurized twin} if the cabin diff, along with cabin alt are not at the EXACT figures given in the POH, at ten thousand feet, then one can have a seal leak or poorly seated DV window along with a leak in a controll run seal, I have yet to see a new pilot on type pick up these small clues on climb out, a tenth of a needle width is all one may get in these cases, however if one checks every five thousand it will soon be obvious that going any higher is not a smart thing to do, but I have seen very few private pilots who seem to pay enough atention to these clues, or pick up on the indications.One country I lived and flew in for a few years a pilot wishing to fly above ten thousand had to visit the decompresion chamber for training every five years, over here, unles one is ex military pilots have no actual experience of hypoxia symptoms. And by the way, on at least one light presurized aircraft the emergency O2 valve can only be turned on from outside the cockpit.