I'm not exactly sure if this is a bug since it happened on vBulletin as well, but, typically, when you highlight text and insert BB code, it'll look like this:
However, if I highlight the text in just the right way -- and I don't know what the "way" is -- it'll appear like so:
Both work, but I'm just curious.
[B]This is bold text.[/B]
[B]This is bold text. [/B]