Adding the quote tests I found another edge case:
[quote author=]This is a quote[/quote]
that is parsed as:
<div class="quoteheader">Quote from: ]This is a quote[/quote</div><blockquote class="bbc_standard_quote"></blockquote>
For the way we do the parsing this is not entirely a bug, probably the intended behaviour, but since I'm not entirely sure, what do you think?