We report measurements below 1.5 K of the residual resistivity ${\ensuremath{\rho}}_{0}$, the temperature derivative of the electrical resistivity, d\ensuremath{\rho}/dT, and the thermoelectric ratio G, for potassium (K) samples encased in polyethylene and Teflon tubing or in contact with either polyethylene or the halocarbons Teflon and Kel-F, and for a dilute K-Rb alloy encased in polyethylene tubing. Pure-K samples encased in any type of tubing show anomalous behavior in ${\ensuremath{\rho}}_{0}$, in which ${\ensuremath{\rho}}_{0}$ is unusually large upon initial cooling of the sample to 4.2 K and then decreases upon ``annealing'' at room temperature to the lowest values characteristic of bare samples. We attribute this behavior to constraints placed upon the samples by the tubing. Both pure-K and alloy samples in contact with polyethylene show anomalous behaviors in d\ensuremath{\rho}/dT and G below 1 K that have all of the characteristics of a Kondo effect, including a resistivity minimum, a thermoelectric anomaly, and disappearance of both effects upon application of a small (\ensuremath{\simeq}0.1 T) magnetic field. No such anomalies are seen in bare unstrained samples or in samples in contact with Teflon or Kel-F. We argue that these ${\ensuremath{\rho}}_{0}$ and Kondo-like anomalies provide plausible mechanisms for a substantial portion of the unusual behaviors that were reported in the first high-precision measurements of d\ensuremath{\rho}/dT in K in the vicinity of 1 K, which had previously been attributed to the presence of either defects or a charge-density-wave ground state in these samples. We have not been able to discover the physical source of the Kondo-like anomalies.