Don't forget to oil your bore after you clean your rifle...
Don't forget to remove all the oil in the bore before you load the rifle!
Don't buy a gun that needs the little red/orange disks, because you will forget them at home or run out, and no store within 30 miles will have any in stock.
Shooting black powder without a bullet seated make a nice deep boom, shooting smokeless (only in a 10ML) without a bullet makes a pretty fireworks show. Unfortunately neither impress deer very much!
If you remember your powder measure, you'll forget the powder can... and visa versa!
Pistol bullets need sabots. If you buy them separately, put them together as soon as you have both or you'll leave one at home!

[:@]
Mike