Combo #65279 is supposed to be the last editable combo, as the combo list ends there. If one scrolls down the end of the combo sidebar (next to the screen editor), the menu stops after while at that "last" combo, as it should. However, if the mouse wheel is moved particularly quick manner, it is possible to have the sidebar skip over the end and stop at a combo a number of rows below the actual "last" combo, letting you edit combos with higher numbers that should be possible.
What's interesting is that these "nonexistent combos" still function like any other. They can still have animation and can be placed on the screen. It makes me wonder where the memory for these is allocated.
EDIT: For what it's worth, combos after the "last" one don't carry-over through a quest save, so they are actually totally worthless.
EDIT2: I just noticed as well that applying the same trick while in combo alias mode crashes ZQuest.