Got the same problem, my guess is that it is the cache, as my problem came after trying to replace a normal link with one limited to 7 posts.
This also explains why [minitwitter username="webdevdesigner" limit=7] gave the tweets from webdevdesigner followed by the tweets from the earlier source I asked for Tweets from.(asked for one, replaced it with the webdevdesigner and saw the ones from the first although the script calling the first was deleted).
Any way to clean the 'mini Twitter feed' cache?
It could also be a conflict from me calling the tweets in the footer with the code <div class="entry">
<div class="tweets_header">Fra Twitter</div>
<div class="content_tweets"> </div>