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If you've ever wondered why I sometimes have tweets with extra spaces in them, I habitually copy as I type as a "backup". When the web app blows up, I paste to "restore" but twitter breaks this. I remove the extra spaces I can spot, but it's hard to spot if it's at a line break
The previous tweet was going to end with an emoji, but inserting the emoji took me from 1 char remaining to -3.
Did twitter switch from characters to bytes and switch to the gauge so that it would be less obvious that they switched? 
I just hit insert once and two appeared.
I ran into one UI issue that's arguably not a bug and three bugs while typing these tweets. How many person-years would you guess went into this text box? In the linked thread, the guy said he worked on this for years and it doesn't sound like he was the only one.
Are the extra 3-4 characters in the pasted text from extra space characters (inserted or appended by Twitter) or Emoji "characters" converted to bytes on the roundtrip?
Extra spaces. Some spaces turn into two spaces (I haven't tried with no spaces to see if spaces get inserted if there were zero spaces).
Some characters count one character to Twitter, some count two; not because of bytes, but because Twitter's rules for ideographic languages and alphabetic languages are different. From my test, emoji also count as two.
Oh, I made a mistake! 
counted as 4 when appended to the tweet mentioned above (also true when I deleted one or two characters) but it's 0xE298B9. If I just paste in a bunch of them, they each count as 2. Seems hard to imagine that the difference is by design and not a bug?
Emoji mostly count as two characters, and some count as more. The England/Scotland/Wales flags count as fourteen each (most flags are four) It's not bytes, though, it's much more complicated. @FakeUnicode and @11rcombs have the details.
These codepoints cost 1/280:
[U+0000..10FF]
[U+2000..200D]
[U+2010..201F]
[U+2032..2037]
All the rest cost 2/280.
1f3f4-e0067-e0062-e0073-e0063-e0074-e007f GBSCT SCOTLAND
7 codepoints all of the 2/280 ilk = 14/280. THANKS TWITTER.pic.twitter.com/LNZuy4OBc0
They wanted to exempt CJK from the higher character count b/c they already had an "unfair advantage" from their higher information density…
…but they half-assed it and caught a whole lot of other stuff. See @FakeUnicode for details
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