[Plugin: TweetMeme Button] TweetMeme button shows ? in only one post

I use the TweetMeme WordPress plugin with code that is manually hardcoded into the template. Occasionally, when I post a new post, the TweetMeme button will show a ?. Clicking on it shows a 404 error — eg., that page does not exist. However, this appears to happen at random to new posts — none of the other posts are affected by this error. Within several hours, the ? goes away, and it appears to fix itself for that one post. Any reason why this happens? While the error fixes itself, I would prefer this not to happen as it occurs at random and seems to be broken for several hours at a time.