When you have kids in your house, especially ones who can reach higher shelves, your stuff is not your own.
I have become irate in more than one occasion because I couldn't find tape when I needed it. The kids can go through a roll of tape in an hour, then when I've got a present to wrap, I am tapeless. So I keep one roll on a really high shelf (which they can still reach) and am very strict about anyone using that tape.
Pens are the other thing that drive me crazy. I can never find a pen when I need one. I used to keep them in my bookcase, but the kids would just take them. They have a million crayons, markers, pens of their own, but somehow mine always get taken. So again, I put two pens on the top shelf of my bookcase and said no one is allowed to touch them. This is difficult to maintain though.
My daughter: "Can I use one of your pens? I want to draw."
Me: "You have a million markers and crayons!"
Daughter: "No, I don't."
Me: "You do!"
Daughter: "Can I just borrow one of them?"
Daughter: "Why not?"
Me: "You won't return it."
Daughter: "Why do you need two?"
Me: "In case one breaks." (or in case someone "borrows" one of them)
Daughter: "Ugh, fine!" [stomps off to her room and returns one minute later with gigantic bucket filled with ten thousand pens]
And don't even get me started on food. Last week, I brought home an entire chocolate cake, and a day later it was simply gone.