  On the way towards quantum gravity and the unification of interaction,
several ideas have been rejected and avenues avoided because they were
perceived as physically unviable. But in the literature there are works in
which it was found the contrary, namely that those rejected topics make sense
after all. Such topics, reviewed in this article, are negative energies
occurring in higher derivative theories and ultrahyperbolic spaces, ordering
ambiguity of operators in curved spaces, the vast landscape of possible
compactifications of extra dimensions in string theory, and quantization of a
3-brane in braneworld scenarios.
