Is this your forum or someone else's forum? This is probably the Bug Fix that you or the person that included the fix tried to add: link.asp?TOPIC_ID=8574
That code I posted is by no means an official fix. A fix has not been incorporated in any base code yet. There will be bugs with it, as you have found. When an official fix has been done, the subject line will normally have BUG+FIX and the thread will be closed and moved to DEV Bug Reports (Closed) Forum
Just delete the 3 lines of code that you added to the top of post.asp if it's giving you an error.