Tweets can sometimes fail when the twitter API is overloaded. This happens very occasionally.

When it does happen the tweet will not go through and it will not appear in your twitter timeline. You will also not receive the 25 credits. 

In order to prevent system abuse and spamming we unfortunately still have to enforce the 7 day delay, so whether your tweet fails or succeeds you will have to wait another 7 days. The reasons for this are technical, and due to the normal round trip latency between twitter and twiends.