Cutting through some of the banter... But the basic facts boil down to the fact that spirits are independent sapient beings. They are capable of acting on their own discretion and as a result are fallible, they can make mistakes. Now, that could be misinterpreting instructions, it could be distraction, it could even be simply not noticing something.
Some of these things could be based on the summoner-spirit relationship, Reaver's example of the spirit following the letter of the instruction but not the intent is just one possibility. A summoner that is on bad terms with spirits might find them doing something like this. You could also use magic to distract a spirit that is standing guard. I believe there is an Astral illusion spell that could be used to hide from a spirit's astral sight. Or even just overloading it with a Mana Static spell.
There is only so much you can load into a single service, so it is impossible to expect to cover every possible loop-hole. And if you're using a spirit to guard just a single door, then you leave every other avenue of entry open to intrusion, free of that spirit's watchful eye.
And all of that is assuming that this door is never used by authorized users. As soon as you have the possibility of use by people that shouldn't cause an alert you open up a whole new avenue of confounding factors.
TL;DR: There's no such thing as a perfect security system.