In Exodus 19, God describes a number of preparations that Moses should make before He will descend to the top of Mount Sinai. One of them is to cordon off an area around the mountain:
You shall set bounds for the people round about, saying, ‘Beware of going up the mountain or touching the border of it. Whoever touches the mountain shall be put to death: o hand shall touch him, but he shall be either stoned or shot; beast or man, he shall not live.’ When the ram’s horn sounds a long blast, they may go up on the mountain.—Exodus 19:12-13 (NJPS)
But three days later, after God appeared on the mountain in fire:
The Lord said to Moses, “Go down, warn the people not to break through to the Lord to gaze, lest many of them perish. The priests also, who come near the Lord, must stay pure, lest the Lord break out against them.”—Exodus 19:21-22 (NJPS)
Naturally, Moses expresses some confusion:
But Moses said to the Lord, “The people cannot come up to Mount Sinai, for You warned us saying, ‘Set bounds about the mountain and sanctify it.’” —Exodus 19:23 (NJPS)
So why did the Lord tell Moses to warn the people not to come near a second time? The text doesn't seem to support the idea that the Lord forgot about the first warning, but that sort of seems like what Moses implied.