Do Not Peep At Cutting Points

Bad peep  

peeps at the cutting point, at which happily connects. A typical thank you move, since White has not accomplished anything while in return Black's major aji has disappeared.

White should cut  

White should cut at in this diagram. Now the group at the bottom does not have a base and is under attack; and can cause trouble later in the game.

Compare the two diagrams. This one is obviously better for White.