I have found that the dreaded Status Code 400 com.facebook.sdk error 5 can relate to duplicate posts of the same content.
I found this when testing an app and posting wall statuses in the background after authorisation.
Simply appending a time() string to the status solves the issue in testing. Obviously you are going to want to get some copywriting skills to make each post unique if posting in the same session.
Subsequent sessions appear to be ok.