The infamous "N-word" (which is so virulent it cannot be even used clinically in many places anymore) used to be common language, even without racist overtones. Racist terminology is also a prime example of this. The advent of social media however has allowed for more push back. note It's actually been that way in the disability activist community since the 'Social Model of Disability' was pioneered by British Disability Rights activists in the 1970s, but frequently actually disabled people get spoken over by educators and media professionals. As of the mid-2010s, the treadmill turned again, and now "disabled" is again the preferred term. The term used to describe people with life-changing diseases or injuries followed a similar path, from "crippled" to "handicapped" to "disabled" to "physically challenged" when terms like "handi-capable" and "differently abled" were proposed, it came across as too clunky and people generally agreed to stop messing with it. After decades of that being used as an insult, "retarded" is now considered so offensive that some people want it classified as hate speech. As these terms fell into common use as insults, they were replaced by a kinder and gentler term: "mentally retarded".
The words "idiot", "moron", and "imbecile" started as clinical terms, referring to people with IQs below 75, 50, and 25, respectively. Some of these examples result from the euphemism treadmill, whereby terms are repeatedly replaced as the previous word falls into such a state of misuse that it cannot be recovered.
note It would be amusing if people from The Gay '90s Time Traveled to today, and wondered why there were people who opposed prostitutes getting married, and happy marriages in general. This means that, for longer than you might think, writers have been employing that word deliberately in order to get crap past the radar. The meaning of the word "gay", for instance, began to change as early as 1870 among the criminal classes of New York, where it originally meant "prostitute" (yes, before The Gay '90s) around 1900 the meaning changed to "homosexual prostitute" and within five years of that to simply "homosexual". Keep in mind that some of these words actually did have their modern meaning at the time they were used, but only within certain sections of the populace. See also Double Entendre or Intentionally Awkward Title for when this trope is invoked entirely intentionally, Separated by a Common Language for the spatial analogue, and Get Thee to a Nunnery for the reverse process.
Compare with Hilarious in Hindsight, of which this is arguably a Sub-Trope.