What string theory forbids

In this article, I would like to collect some of your examples of phenomena or situations that are completely compatible with low-energy effective field theory but do not arise from any known - and perhaps not even any unknown - stringy compactication. In other words, predictions of string theory that are independent of the vacuum selection mechanisms.

For example, pure N=2 supergravity is anomaly free and OK as an effective field theory, but string theory always predicts new multiplets, does not it? Also, the U(1)^{496} supersymmetric Yang-Mills theory coupled to type I supergravity in ten dimensions is anomaly free and nice, but it does not seem to describe any stringy background.

There are other similar properties related to the axion decay constants and the strength of various gauge couplings that apparently can't be anything you want; work in progress. However, I would love to hear some completely different examples of universal predictions of string/M-theory. Thanks.