In the bad old days, people referred to those with mental disabilities as "subnormals." And institutions where they were treated / looked after were sometimes referred to as "Subnormality Hospitals." Not a very nice phrase, and moreover, discredited, because having a mental disability does not necessarily mean they're stupid. Similarly, people with Down's Syndrome were referred to as "mongoloid." Cue every schoolchild in the land calling their compatriots who do something stupid or clumsy as a "mong."
Then they started being called "spastics." There was a charity called The Spastics Society in Britain here. Result - "spaz" or "spacker" was the schoolchild's insult of choice. With the then recent appearance on Blue Peter of Joey Deacon, it became invariably accompanied by a chorus of, "nnnnnngh!" and a show of belming. Belming, incidentally, is where you stick your tongue between your teeth and your lower lip to make yourself look sort of like Joey Deacon.
Then The Spastics Society renamed itself to Scope because their name had become an insult. All of a sudden, any kid who does something stupid or clumsy is referred to as a "scoper." I was once a scoper when I completely allowed the ball to slip through my fingers while on a clear run to the tryline at school-mandated rugby. I admit it, it was a total upfuck because I, who was large but ungainly and not very good at running, was so excited about actually being able to score a try that I dropped the ball like a clumsy oaf, and by the time I'd realised and turned to regain it, a lad called Richard Ralphs (known as "Mouse" because he was small but stupidly fast on his feet) had beat me to it and was off back down the field like a shot.
Meanwhile, a disabled person in general might be referred to as having "special needs," or when they do something like appear in public with their disability, they get referred to condescendingly as "brave." Now "special" and "brave" are derogatory terms for disabled folks. "He's a bit special," they say of someone who has a mental disability. I had one client who was disabled and in a wheelchair and the same age as myself, and he utterly hated being referred to as "brave." I can't say I blame him; it's like he's damaged goods or something, referring to the man as "brave."
This is a perfect example of the euphemism treadmill, where terms for a variety of person who might be viewed negatively by the population at large are co-opted as terms of abuse. The Powers That Be realise this, and rebrand themselves, or the term of that class of person, or encourage the use of alternative terms. However, this does not remove any underlying prejudice against them and so the new euphemism becomes a term of abuse as well. And it is a treadmill because within a set period of time, the new euphemism finds itself used insultingly.
This also happens in Japanese, where it is referred to as "kotobagari", or "word-hunting," and leads to some incredibly convoluted turns of phrase.