If you indeed took your time (as opposed to rushing), then rest assured this is a visualization error that can be corrected with a lot of puzzles/tactics exercises. In time, most chess players ramp up on visualization with nothing more than playing tons of games, so don't worry about these "vision blindspots" too much.
You can try some of these exercises out ... they really help!
http://danheisman.home.comcast.net/~danheisman/Articles/Exercises.html
Thanks for the advice and comfort everyone. I'll take a look at those exercises, and I'll work with the Tactics Trainer here on chess.com.
Rooks often live on the back rank; if you actually lift a rook off of it, you're always risking that you're hanging the other 
. So always keep that in mind when you do that. And also, even if there is no other rook on the back rank, the back rank itself could be vulnerable to annoying checks there, depending on how much room you made for your king.
I just never [i]see[/i] these things.
I sat there and thought about that final move of mine for a [i]long time[/i], guys.