Do you know about the BBcode editor? Chances are you have some stray bit of code that needs moving. If you look on the upper right when you are in editing or making a post you will see and icon that looks like a box (there are two icons, the innermost one). Click on that and you will see the code and can check to make sure all is where it should be. I used to have the worst time with fieldboxes until I figured that out.
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. ^^