Twitter has said that there are working on a fix on their status page. On their know issues page they have announced that they are aware of tweets from strangers in users Timelines and are working to fix the problem.
If this is a problem you face and are wondering what to do. Here's what Twitter says on their page:
What to do:
We would greatly appreciate information about your situation if you are affected by this issue. Please include the following information in the comments section below:- Your username (Please note that usernames do not contain spaces)
- The username of the stranger in your Home timeline
- A link to the tweet from the stranger
- A link to a screenshot of the tweet from a stranger in your Home timeline (post the image on a photo-sharing website)