Actually, this one is pretty hard to screw up, but if you tried hard enough you’d break something expensive. Owing to the design of the newer 6500-series chassis, you can’t put the Supervisor blades anywhere, at least not the 720. If you look at the back of the module, you’ll see why. The obvious part is that the structure with pins covers nearly the whole back side of the module — if you look in the 6500 chassis, there will only be two slots that match. The others will have a gap on the left, and the black plastic mating medium start at about 25% of the length and goes close to the end. The other thing you’ll find, if you look closely, is that there is a crescent-shaped slot on the 720 module, and a matching rod maybe an inch long with a corresponding crescent-shape (half circle, more precisely) in the back of the correct 6500 slot. The orientation of that little peg is the same for the supervisor (or switch fabric) slots, but reversed from all the others. Despite the small size, this pin prevents you (in normal circumstances) from pushing the module in the wrong slot far enough to damage it.
I was used to putting the Sup at the top of the slots, or in some cases the bottom. But in a 6509, for instance, those puppies are going to go in the middle. If you’ve got redundant supervisors for the 6509, that would be slots 5 and 6. Note that this isn’t the same as you would find in the 6513 or 6506, and obviously not the same as the 6503 either.
Here’s the run down:
Occupies the switch fabric slots in the chassis:
3-slot chassis—slots 1 and 2
6-slot chassis—slots 5 and 6
9-slot chassis—slots 5 and 6
13-slot chassis—slots 7 and 8
That info is from a pdf document, CISCO CATALYST 6500 SERIES SWITCH SUPERVISOR ENGINE 720 .
And there you go.