Here is a sentence whose punctuation poses an interesting grammatical problem, and where there are problems, of course, there are opportunities:
In fact, they behaved like an authoritarian dictator: questioned people on the street with whom they disagreed, detained people with whom they disagreed, forced out people with whom they disagreed.
The author wants to criticize the behavior of protestors who are protesting the actions of a government, and he is worried that good intent can often quickly devolve into behavior as contemptible as that which it is trying to correct. The grammatical problem lies in the constructions that follow the colon.
The colon and semicolon are what we could call composite marks of punctuation: we stack a period onto another period to build the colon, or onto a comma to build the semicolon. As composites, they are strong and durable, certainly stronger than the simple comma, but they don’t quite yet possess the singular strength of the diamond-like period. This is important to realize because it will help determine the grammar that follows these marks.
In the sentence we are looking at, the author has wanted to follow a triadic structure after the colon: three clauses that each begin with a verb whose subjects have been suppressed (that is, not directly stated): questioned, detained, forced out. Using an elliptical style, he is expecting the reader to supply the pronoun they from the clause before the colon; they who behaved like an authoritarian dictator are the same ones who questioned, detained, and forced out.
Ellipsis—the omission of words you expect the reader to easily assume—is a recognized and effective stylistic device, but it can be tricky to deal with at times. One of those times is often (not always) after these composite punctuation marks. The colon and semicolon act as breakwaters, structures that control the flow of meaning as the reader is carried through the sentence; without them, the clear-cut clarity we expect of our modern prose would roll and wheel and whirl on and on; our sentences would never really organize our thoughts for the reader in a manageable way.
But as a breakwater, the colon (let’s limit our discussion to the mark that appears in the example) has effectively prevented the pronoun they from reaching the three verbs after it; on the other side of this punctuating structure, the reader does not immediately (and there’s the key that always justifies ellipsis) understand who questioned and detained and forced out. The semicolon has checked the flow.
So how change things? The obvious move would be simply to supply the pronoun again before each of the three verbs (they questioned, they detained, they forced out), but the problem with that would be just that: it’s too obvious. Instead, we have here an opportunity to change the three clauses into phrases, and so tell the reader what they did (not how they acted) when they behaved like an authoritarian dictator: questioning, detaining, and forcing out people with whom they disagreed.
The caution, then, is to remember that marks of punctuation like the colon and semicolon create new patches of syntax that are directly related logically to what precedes them, but not always grammatically. As always, an eye to the structure will help ensure a comprehensible and attractive design.