12.) The terminating of more than one grounded conductor under a single terminal.
Grounded Conductor Terminations 408.41
It is not uncommon to find the termination of more than one grounded conductor under a single terminal, in a panelboard. The NEC, as well as the manufacturer, limits terminating the grounded conductor in a panelboard to a single conductor per terminal. Unless you are dealing with parallel grounded conductors under a terminal designed for the purpose, each grounded conductor shall terminate within the panelboard in an individual terminal that is not used for another conductor.