Nikon D90 AF issue, any

The AF will attempt to focus and blink (meaning its ready and in focus). However, randomly, the viewfinder isn't actually focused, and while the AF says it's in focus, it's actually extremely blurry. It happens every now and then but I want to be sure my camera is OK. I've taken very good care of my camera and it has never been dropped or anything like that. What could cause that?

Added (1). Additionally it will take a blurry photo. It seems to really "think" it's in complete focus. Again this doesn't happen much, but I'm just neurotic! Lol.

Added (2). No, the photo when I actually take it, isn't in focus after it's been taken. The photo is completely blurry.

Let me see if I understand what you're saying, the viewfinder says the photos are blurry but the actual photos are in focus?

If so, then your dioptric adjustment knob has been fiddled with (this can happen from something as simple as brushing against it). You just need to readjust it, it's not that big of a deal. I have to do it practically every two weeks. Have a look at this: http://digital-photography-school.com/using-diopter-adjustment

Snorlax is right, BUT if you check the image on the LCD or on a computer and its still blurry, then there's somethign wrong with tha focusing.

take it to a camera store… But don't worry, it happens to me sometimes, but it doenst effect me…