An interesting Language Log discussion focuses on the word diaper and its tendency to be used in the plural:
Viewed historically, diaper and nappy were originally construed as singular, but plurally marked diapers and nappies in singular contexts became more frequent by the mid-20th century. An example from 1960 appears in the OED draft entry for mess, taken from A.S. Neill’s Summerhill (a popular account of Neill’s pioneering Summerhill School). The quote voices a boy’s thoughts about his younger brother: “If I am like him and mess my trousers the way he dirties his diapers, Mommy will love me again.” (That’s from the U.S. edition — the U.K. edition, reprinted here, has nappies instead of diapers.) The parallel structure here is telling: “mess my trousers” vs. “dirties his diapers/nappies.” The plurally marked diapers and nappies appear to be influenced by pants and trousers — words that almost always appear in the plural, or pluralia tantum as they’re technically known…
Even though diapers and nappies have gone a long way to joining the pants family, they remain something of a special case since they’ll never be pluralia tantum. When it’s not worn, a diaper is just a diaper: a piece of fabric with no leg-holes. Only when it’s worn and transformed into something “pants-like” can all of those -s forms exert their analogical influence, leading to a preference for diapers over diaper. But it remains only a preference, since even when worn a diaper can still be construed singularly.
There is further discussion of U.K. and Australian usage, and of distinctions between cloth diapers and disposable diapers and between infant diapers and “pullup” diapers, not to mention diaper covers. Anyone who has had occasion to discuss diapers is welcome to weigh in with their own usage.