Stupid question, are you not able to isolate in the current theme where the error occurs??
You say its before every post "to" your blog. Do you mean before every post that is displayed on your blog?
If thats the case, its something in the loop thats occurring before
the_content (my guess based on your vague info) , in which case you need to simply compare the theme you are trying to use with a theme that doesnt produce that error.
Some themes havent been updated to be compatable with 2.3.x.. so you crack open the files and you make the changes.
In reality, you havent provided enough info for much more of a suggestion, before a post is vague .. does a post include the meta info, or just the content of the post itself.. etc..
I dont know.
Lastly, and most importantly, this forum has a wonderful search feature, and very little hasnt already been covered. Its always a good idea to put error messages through the search.