When you're not currently in an animation (because there's nothing to buffer from), when you're getting grab-released (but not when you grab-release someone), and when you're on the ledge before you can do an option from the ledge.
Also, if you get grabbed while pressing buttons prior, those commands don't count as "buffered mashing" inputs.
Also, if you get grabbed while pressing buttons prior, those commands don't count as "buffered mashing" inputs.