John Calvin Commentary Psalms 64:8

"So they shall be made to stumble, their own tongue being against them: All that see them shall wag the head." — Psalms 64:8 (ASV)

And they shall make their own tongue to fall upon themselves. Pursuing the same subject, he remarks that the poison concocted in their secret plans, and which they revealed with their tongues, would prove to have a deadly effect upon themselves. The sentiment is the same as that expressed elsewhere by another figure, when they are said to be caught in their own snares and to fall into the pit which they have digged themselves (Psalms 57:6). It is just that Heaven should make the mischiefs which they had devised against innocent and upright people recoil upon their own heads.

The judgment is one that we see repeatedly and daily exemplified before our eyes, and yet we find much difficulty in believing that it can take place. We should feel all the more bound to impress the truth upon our hearts, that God is always watching, as it were, for His opportunity to turn the stratagems of the wicked into means for their destruction, just as completely effective as if they had intentionally employed them for that purpose.

At the end of the verse, to point out the striking severity of their punishment, it is said that all who saw them should flee away. The judgments of God are often beyond the sight of an ignorant world, and before it can be roused to fear and dismay, these judgments must bear striking signs indeed of a divine hand.