Exegesis:

pas ho pesōn ep’ ekeinon ton lithon sunthlasthēsetai ‘every one who falls on that stone will be dashed to pieces.’ The clause is an expansion of v. 17 but it changes the picture considerably since it refers no longer to a cornerstone, but a stone which is in such a position that one can fall upon it, or, as the rest of v. 18 shows, it can fall upon somebody.

sunthlaō ‘to crush together,’ ‘to dash to pieces,’ here of people. eph’ hon d’ an pesē, likmēsei auton ‘(the man) on which it falls, him it will crush.’

likmaō ‘to winnow’ (so usually in Greek), hence ‘to scatter (like chaff),’ or ‘to crush to powder,’ ‘to pulverize,’ preferably the latter.

Translation:

The translator should not feel called upon to set right the incongruity that exists between v. 17 and v. 18 (as shown in Exegesis).

Every one who … may better become ‘when a person … he,’ cf. on “whoever…” in 9.24.

Will be broken to pieces, or, ‘will go to (or, break into) pieces,’ ‘will shatter himself on it’ (Bible de Jérusalem). ‘will break his body’ (Sranan Tongo).

When it falls on any one it will crush him, or, ‘the man whom the stone falls upon, that one it will smash completely’ (Sranan Tongo); or, keeping the same subject, and using a passive form of the first verb, or of both, ‘he who is-fallen-upon by it (or, by the stone) will turn-to-pulp, or, will be shattered’ (Javanese, Malay).

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.