In the text editor, click the little box in the upper right corner that says "Use BB Code Editor."
This will take you to the plain-text editor where you can edit out the offending code.
In a nutshell, this error pops up when you use the formatting tools in the Rich Text Editor (like colors, bolding, fonts, etc), over part of the BBCode tags but not over the entire BBcode tag.
The forum gets confused and tries to be helpful by closing the tags for you, not realizing it's actually messing things up further. But if you go into the BB Code Editor and just edit them out, you should fix your problem. ^^