These kinds of isolated surrogate pairs are pretty easy to create if you're doing the right kind of processing on the right kind of data.
Suppose you receive a long piece of text wrapped in JSON, unpack it into a JS String, then start processing it in fixed size chunks. If your source text contains any significant percentage of surrogate pair-represented characters, you'll eventually break one.
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These kinds of isolated surrogate pairs are pretty easy to create if you're doing the right kind of processing on the right kind of data.
Suppose you receive a long piece of text wrapped in JSON, unpack it into a JS String, then start processing it in fixed size chunks. If your source text contains any significant percentage of surrogate pair-represented characters, you'll eventually break one.