I note that most common glitches of this ilk happen when an illogical sequence has been done by my disembodied fingers, as when [/i] (using '<'s, of course) precedes a [i].
But I think my usual Error is not in the 500 Series (I don't often read the barfle-text for comprehension.)

Safari keeps a copy of the text, so no more lost brilliance when I go back to fix (no need to compose in another editor, that is.)

Pity we have no briefer subs for 'blockquote' and other frequent entries.
Want a KB with 256 soft keys, illuminated; touch a key and a voice tells its function. That's all I want.