I'm unsure this last sentence is correct. Characters aren't utf8 or utf16, and "accidentally cut in pieces" isn't quite documentation material. I think:

```D
/++
$(D startIdx) represents a codeunit index in $(D s). If the sequence
starting at $(D startIdx) do not represent a well formed codepoint,
then a $(XREF utf,UTFException) may be thrown.
+/
```

@jmdavis, is this correct wording?