Here's one for you.  Not my forum, but one I help moderate. 

When replying, or quoting it keeps adding a "Re:" to the subject line.  so ending up with the subject like this for example,  Re:   Re:   Re: Site changes  A screenshot is included below.

Any idea what file(s) and where to look to find the problem.  This site is currently running 2.0.13, but will update to 14 soon.

Mods installed are,

. Spam Blocker 1.0
. SMF Topic Prefix Mod 1.2.2
. Topic Solved 1.1.1
. Country Flags 1.1.2