I don't know if this is a bug so much as a suggestion for programming. Namely the following rule to be created: (sorry, very awkward to describe) If a closing "]" is missing from external url syntax, then when the code is previewed (or viewed as normal), an opening "[" will be shown as "[" ...if the opening "[" is not followed by a closing "]" prior to a double return or a heading or a bullet or a line ....
Sterling
Let's just hope we get somewhere with the new parser, so we can stop having to think of horrible special cases like this - the definition of an ext. link can simply not be met if there is a new paragraph, or heading, or whatever...