Romans 2:17-21 and 5:12 appear to be examples of aposiopesis (ἀποσιώπησις). David A. Black explains that this particular rhetorical device "is a deliberate failure to end a sentence under the influence of a strong emotion like anger or fear, as in 'If you do this, I'll . . .' Aposiopesis is seen in Luke 19:42" (Linguistics for Students of New Testament Greek, page 134).
Another well-known (possible) example of aposiopesis that comes to mind straightaway is Gen. 3:22.
See the informative page here: http://grammar.about.com/od/ab/g/aposioterm.htm