  The geometry dependence of Casimir forces is significantly more pronounced in
the presence of thermal fluctuations due to a generic geometry-temperature
interplay. We show that the thermal force for standard sphere-plate or
cylinder-plate geometries develops a non-monotonic behavior already in the
simple case of a fluctuating Dirichlet scalar. In particular, the attractive
thermal force can increase for increasing distances below a critical
temperature. This anomalous behavior is triggered by a reweighting of relevant
fluctuations on the scale of the thermal wavelength. The essence of the
phenomenon becomes transparent within the worldline picture of the Casimir
effect.
