  Kronheimer-Mrowka recently proved that the Dehn twist along a 3-sphere in the
neck of $K3\#K3$ is not smoothly isotopic to the identity. This provides a new
example of self-diffeomorphisms on 4-manifolds that are isotopic to the
identity in the topological category but not smoothly so. (The first such
examples were given by Ruberman.) In this paper, we use the Pin(2)-equivariant
Bauer-Furuta invariant to show that this Dehn twist is not smoothly isotopic to
the identity even after a single stabilization (connected summing with the
identity map on $S^{2}\times S^{2}$). This gives the first example of exotic
phenomena on simply connected smooth 4-manifolds that do not disappear after a
single stabilization.
