Fluid bathes the outer and inner surfaces of the cell membrane.
You might think the cell membrane is rather solid, but it is actually more fluid-like. Instead the cell surface seems to flow over and seal the puncture. It has to do with the properties of lipid molecules. The reason the puncture automatically seals itself is because the puncture is energetically unfavorable because it leaves too many hydrophobic groups exposed. The membrane is also asymmetric, which means the inside of the CM looks different than the outside.