First, sorry for my poor English, I'm french and doing my best
Now to the bug : if you select the first or the two characters of a post (wathever they are, even part of a word or spaces), then left clic with mouse the B style (for example) for bolding this character (or these two characters), all the message (wathewer his length), will be bolded.
So, in this special case, the close of the bold is misplaced and set at the end of the message instead of being placed after the first character (or after the second character).
Hope this help ! And thanks for the great job on phpbb !