I am interested in understanding to what extent continuous bijections fail to be homeomorphisms. For example, suppose $X, Y$ are metric spaces and $f: X\to Y$ is a continuous bijection. Is it possible that $f^{-1}$ fails to be continuous at uncountably many points of $Y$?
Apologies if the question is trivial!
Let $X$ be the reals with the discrete topology and $Y$ the reals with the usual topology, and let $f$ be the identity map; then $X$ and $Y$ are metrizable, $f$ is a continuous bijection, and $f^{-1}$ is nowhere continuous.