Is it permissible to make a table for a television set with the possibility that it might be used to watch what is impermissible?

If you are certain or it is highly probable that your work will be used for something Allah has forbidden, then it is not permissible for you to assist. This is because it falls under cooperating in sin and aggression, which is forbidden by Sharia. Examples include selling grapes to someone who will use them to make wine, or selling weapons to someone who will fight Muslims with them. However, if you do not have such a strong presumption, there is no objection to providing assistance.