Contracting a contractible set in $\mathbb R^2$
This is a special case of a more general theorem due to Moore, whose proof should be in the book R.Wilder, "Topology of manifolds". Dimension 2 is very different from higher dimensions (2-dimensional homology manifolds are topological manifolds). Bing (I think) proved that contracting a wild arc in $R^3$ results in a space which is not a manifold.