There's always the remote chance a part of your deleted files will be long and the new message that takes over that sector will be short, hence blocking that space from getting deleted.

With enough bad luck, if the new message never gets deleted, the juicy stuff you wrote just happen to be near the end of the block sector (which you can't control for since you don't know the remote sector size [usually but not always 512 k], the length of the meta data) and the investigators luck out and find enough to identify that it was you....

well, lets just say you're screwed