Asklemmy

I used to say "worth nothing" while, obviously, the correct way is "worth noting".

However, given how many Google results are there about the wrong spelling, I'm clearly not the only one.

I don't use it wrong because I don't use it but to me "mirth" feels like it has a negative connotation even though I know it means joyful.

Turns out pseudo is not spelled like sudo.

When I was a kid, it was Yosemite.

I was obsessed with GTA San Andreas, and that was the big truck in the game, and it was my favorite. I was pronouncing it like "yosa might" for a while until somebody pointed it out, and then I connected the dots

Nonce. I've always used it just like Dunce. Turns out it does in fact not mean the same and instead means pedo.

I have bad hearing so I mostly just mispronounce words...

In German we use the English term 'understatement' in some occasions, e.g. 'his dressing style is a clear understatement...'. My brain somehow tied the meaning to something like 'understanding', maybe due to the similarities of both words. For decades it was clear to me that someone dressing like that were dressed to the point and 'making a clear statement'. Now that I've checked the real meaning, I'm completely puzzled when and how to use the term and what I've misinterpreted all the years...

The word Segue meaning transition.

Always used Segway instead which was incorrect.

Encephalitis (en-sef-uh-LIE-tis) is inflammation of the brain. There are several causes, including viral infection, autoimmune inflammation, bacterial infection, insect bites and others. 

Proscribe and prescribe.

Penultimate. I used it as though it referred to the last thing rather than the second to last thing.

Inflammable means flammable? What a country

Since OP is in the UK, I can pull out “nonplussed.” Current American usage of the word is a lack of surprise or general acceptance. I am nonplussed when news arrives that another politician was caught in a sex scandal. Non-American usage is complete surprise and an inability to act. The Scot was nonplussed when the drunk American vomited noisily on his shoes.

Edit: I am firmly in the “general acceptance” camp and usually have to process for a second or two when someone uses it in its traditional sense.