When considering this question one has to remember the basis of the scientific method: formulating hypotheses that can be disproved. Those hypotheses that are not disproved are thought to be true until disproved. Since it is more glamorous for a scientist to formulate hypotheses that it is to spend years disproving existing ones from other scientists and that it is unlikely that someone will spend enough time and energy trying to disprove his/her own statements, our body of scientific knowledge is surely full of statements we believe to be true but will eventually be proved to be false.
So I turn the question around: What scientific ideas that have not been disproved, do you believe are false.
In my field (theoretical economics), I believe that most ideas taught in economics 101 will be proved false eventually. Most of them would already have been officially defined as false in any other more hard-science, but, because of lack of better hypotheses they are still widely accepted and used in economics and general commentary. Eventually, someone will come up with another type of hypotheses explaining (and predicting) the economic reality in a way that will render most existing economics beliefs false.