placing a note outside of the x=0-512 y= 0-512 playfield is impossible (save for a specific very wacky circumstance i will explain later) as when a map is compiled, it automatically snaps the notes to the grid. that said, the grid is actually x=0-512 y=0-384 (something around 380 anyway). by editing a notes y value in .osu to be greater than 384 (say 512), you can move it somewhat off the grid and even offscreen completely. if using cs7, you're actually able to make it so far offscreen that the approach circle never appears either.
so, whats that very specific wacky circumstance? why, its NaN. by editing a note in .osu to have a x or y value of NaN, the object is offscreen. in fact, its around -2147483648 osu pixels offscreen. this is so far offscreen that, if any object draws follow points to said object, the game will lock up and crash immediately. its also so far offscreen that any slider body is horribly, completely, and utterly distorted to the point of complete unusability.