It’s well known that Middle English /ɛr/ became more open in quality and merged with /ar/, while also in some dialects remaining as a distinct phoneme, but now pronounced /ɜː(r)/, leading to alternative forms for many words, of which one has remained as standard, the other becoming merely dialectal or regional: market, but merchant; person and parson; Derby as /ˈdɑːbi/ in British English but /ˈdɜːrbi/ in American English (and also sometimes regionally in England); likewise clerk as /klɑːk/ or /klɜːrk/ (/klɛrk/ still in Scots) – and spelt Clark as a family name; vermin (standard) but varmint (non-standard).
Many younger people in England are now pronouncing /ɜː/ with a much more open quality, so that we may be seeing the beginnings of a new merger, of /ɜː/ and /ɑː/ – unless /ɑː/ also changes its quality.
One oddity is a word that has retained both pronunciations, becoming specialized in its meanings while at the same time, in a sense retaining the original meaning: sherd and shard (unlike person/parson, where the two meanings are well differentiated). The glass spike sticking up above the London skyline is called The Shard, and a shard is more often than not a piece of broken glass, whereas a broken piece of pottery is a potsherd. When I first saw this word, I misdivided it, and assumed it was ‘pots-herd’, and I couldn’t understand how you could herd pottery (I was very young at the time). I’m sure that part of the reason I didn’t properly understand the word was that in The Potteries, a rubbish heap of broken pots, which every factory (or potbank) inevitably created as there is always a percentage of waste in the firing, is known as a ‘shordruck’. I never saw the word written down, and as Staffordshire has long been a non-rhotic accent, had assumed as a child that it was spelt ‘shawdruck’, in which I also mis-placed the juncture, making it ‘shaw-druck’. The OED does give shord as a historical dialectal variant of shard/sherd, but doesn’t recognise its current use, saying that it is found in the 1700s and 1800s, and gives only one (modern-ish) quotation, from R D Blackmore (best known for his novel Lorna Doone), dated to 1882. The ‘ruck’ half of this word is another story, perhaps for a later date.