I came across a panel that has been serviced by at least six people, some of them Gen. contractors, some even less qualified. The panel has a lot of mysteries because of repeated remodels and renovations to the house.
I'd like to be able to recognize the usual and legal uses of tying breaker handles together. But I would also like to be able to spot "cheats" if any exist.
Multi-wire circuits come to mind, when one duplex has both circuits.
On an unrelated note. I am pretty tired of coming across tiny 10/20 sub-panels maxed out with half-size dual breakers.