anaphoric clogs

in the past week i have seen two different signs, in two restrooms separated by several hundred miles, one typed and one handwritten, that both said:

Do not put paper towels in the toilet!!!
It clogs them up.

wait, what? that should be the other way around: They clog it up. the paper towels are the clog-ers, not the clog-ees. how did such a complete mix-up escape from the pens of two people who are presumably competent speakers of English, if not native speakers?

it requires some charitable grammatical analysis, but i think there could be a few contributing factors. first, it is probably intended as a propositional anaphor, referring to the act of paper-towel-putting. one way to render the intended meaning is It clogs it up, which is a little confusing, since there are two occurrences of it—one propositional and one referential. this is probably the moment during the composition of this sentence when panic set in, and—with a gentle semantic push—the second anaphor became them. despite the unambiguously singular antecedent the toilet in the previous sentence, what the sign-writer means to convey is that paper-towel-putting leads to toilet-clogging in general. that generic reading involves (potentially) multiple toilets, and hence them.

the misfortune lies in the fact that it clogs them up is such a short sentence that the reader can latch on to them before they've even decided that it should be a propositional anaphor. in that case, both get interpreted referentially, with the strange consequence that paper towels are being clogged up by toilets. that would be quite the plumbing problem.