You can't fire fireballs when you're close to, but not actually, above the level boundary.
There appear to be some issues with sprite spawning when using the Autoscroll Special 4 command (might just be my rom getting wonky) and fireballs will sometimes disappear instead of bouncing. In addition, when nearly offscreen, a block's shatter sprites appear at the right hand side of the screen instead of where they're supposed to.
Dry bones (both variations) and bony beetles won't despawn if they have been jumped on and haven't reformed yet. This applies to being off-screen to the left, right or bottom.
This is mostly useful for filling up sprite slots.
Another thing that should be known is that dry bones / bony beetles that have been jumped on and haven't reformed yet hurt Mario only if he's riding Yoshi, but I couldn't find it on the list.
The big boo boss's off-screen check seems inconsistent, upon defeat, if scrolled off-screen, it trigger the end level timer prematurely instead of waiting till it falls below the level. However, while the sprite is active and not defeated, it does process off-screen.
-------------------- Give thanks to RPG hacker for working on Asar.