The figures seem to be taken from the work of building up and pulling down houses. In their general application, they recommend the man who would act wisely to be cautious when taking any step in life which involves risk.

Breaketh an hedge - Rather: "breaks through a wall."
Serpent - The habit of snakes is to nestle in a chink of a wall, or among stones (compare Amos 5:19).

Whoso breaketh a hedge, a serpent shall bite him - While spoiling his neighbor's property, he himself may come to greater mischief: while pulling out the sticks, he may be bit by a serpent, who has his nest there. Some have supposed that נחש nachash here means a thorn; perhaps from the similarity of its prick to the serpent's sting. He who forces his way through a hedge will be pricked by the thorns.

He that diggeth a pit shall fall into it,.... This and the three following clauses are proverbial expressions, teaching men to be wise and cautious, lest by their conduct they bring mischief upon themselves; as it often is, the one that digs a pit for another, falls into it himself, as the wise man's father before him had observed, Psalm 7:15; as kings that lay snares for their people, and subjects that plot against their sovereign; or courtiers that form schemes for the rain of those that are in their way; or any man that devises mischief against another, frequently so it is, that the same befalls them; as Haman, who prepared a gallows for Mordecai, was hanged on it himself;
and whoso breaketh an hedge a serpent shall bite him; which often lies hid in fences, in old walls, and rotten hedges (s), Amos 5:19; so he that breaks down the hedges and fences of kingdoms and commonwealths, and breaks through the fundamental laws of a civil constitution, and especially that transgresses the laws of God, moral or civil, may expect to smart for it. Jarchi interprets this hedge of the sayings of their wise men, which those that transgress shall suffer death by the hand of heaven: but it would be much better to apply it to the doctrines contained in the word of God, which are a hedge and fence to the church of God, and whoever transgress them will suffer for it; see 2-John 1:8; The Targum, by the "serpent", understands an ungodly king, who bites like a serpent, into whose hands such transgressors shall be delivered: and some have thought of the old serpent the devil, as Alshech, who deceived Adam and Eve.
(s) Nicander apud Bochart. Hierozoic. par. 1. l. 1. c. 4. col. 26.

The fatal results to kings of such an unwise policy; the wrong done to others recoils on themselves (Ecclesiastes 8:9); they fall into the pit which they dug for others (Esther 7:10; Psalm 7:15; Proverbs 26:27). Breaking through the wise fences of their throne, they suffer unexpectedly themselves; as when one is stung by a serpent lurking in the stones of his neighbor's garden wall (Psalm 80:12), which he maliciously pulls down (Amos 5:19).

"He that diggeth a pit may fall into it; whoso breaketh down walls, a serpent may sting him. Whoso pulleth out stones may do himself hurt therewith; he who cleaveth wood may endanger himself thereby." The futures are not the expression of that which will necessarily take place, for, thus rendered, these four statements would be contrary to experience; they are the expression of a possibility. The fut. יפּול is not here meant as predicting an event, as where the clause 8a is a figure of self-punishment arising from the destruction prepared for others, Proverbs 26:27. Sir. 27:26. גּוּמּץ is, Proverbs 26:27, the Targum word for שׁחת, ditch, from גּמץ = שׁוּח, depressum esse. גּדר (R. גד, to cut), something cutting off, something dividing, is a wall as a boundary and means of protection drawn round a garden, vineyard, or farm-court; גּדר פּרץ is the reverse of פּרץ גּדר, Isaiah 58:12. Serpents are accustomed to nestle in the crevices and holes of walls, as well as in the earth (from a city-wall is called חומה and חל); thus he who breaks into such a wall may expect that the serpent which is there will bite him (cf. Amos 5:19). To tear down stones, hissi'a, is synon. of hhatsav, to break stones, Isaiah 51:1; yet hhotsēv does not usually mean the stone-breaker, but the stone-cutter (stone-mason); hissi'a, from nasa', to tear out, does not also signify, 1-Kings 5:18, "to transport," and here, along with wood-splitting, is certainly to be thought of as a breaking loose or separating in the quarry or shaft. Ne'etsav signifies elsewhere to be afflicted; here, where the reference is not to the internal but the external feeling: to suffer pain, or reflex.: to injure oneself painfully; the derivat. 'etsev signifies also severe labour; but to find this signification in the Niph. ("he who has painful labour") is contrary to the usu loq., and contrary to the meaning intended here, where generally actual injuries are in view. Accordingly בּם יסּכן, for which the Mishn. יסכּן בּעצמו, "he brings himself into danger," would denote, to be placed in danger of life and limb, cf. Gittin 65b, Chullin 37a; and it is therefore not necessary, with Hitzig and others, to translate after the vulnerabitur of Jerome: "He may wound himself thereby;" there is not a denom. סכן, to cut, to wound, derived from סכּין (שׂכּין), an instrument for cutting, a knife.
(Note: The Midrash understands the whole ethically, and illustrates it by the example of Rabsake we know now that the half-Assyr., half-Accad. word rabsak means a military chief], whom report makes a brother of Manasseh, and a renegade in the Assyrian service.)
The sum of these four clauses is certainly not merely that he who undertakes a dangerous matter exposes himself to danger; the author means to say, in this series of proverbs which treat of the distinction between wisdom and folly, that the wise man is everywhere conscious of his danger, and guards against it. These two verses (Ecclesiastes 10:8, Ecclesiastes 10:9) come under this definite point of view by the following proverb; wisdom has just this value in providing against the manifold dangers and difficulties which every undertaking brings along with it.
(Note: Thus rightly Carl Lang in his Salom. Kunst im Psalter (Marburg 1874). He sees in Ecclesiastes 10:8-10 a beautiful heptastich. But as to its contents, Ecclesiastes 10:11 also belongs to this group.)
This is illustrated by a fifth example, and then it is declared with reference to all together.

An hedge - Whereby another man's fields or vineyards are distinguished, that he may either take away their fruits, or enlarge his own fields.