Stresses which build up in thin films such as tantalum, during thermal processing, can cause major reliability problems in electronic and x-ray optics applications. We demonstrate that 50-200 nm thick sputtered beta-Ta thin films undergo repeated compressive stress increases when thermally cycled to 400-degrees-C (at a rate of 10-degrees-C/min) and back in a purified He ambient because of small amounts of oxygen gettered by the tantalum. The oxygen contamination results from the poor quality of the atmospheric seal on the quartz annealing chamber. As-deposited Ta thin films have a compressive stress ranging from - 1 to - 4 GPa. The compressive stress buildup was monitored in situ and was shown to increase - 0.5 GPa on average after each thermal cycle for a final value from - 6 to - 7 GPa after seven cycles. After being cycled thermally seven times any perturbation of the film such as a four-point probe resistivity measurement can cause the film to instantaneously crack in a serpentine pattern, relieving the large compressive stress. Auger electron spectroscopy depth profiling analysis was used to determine that the as-deposited film contained 1 at. % oxygen which increased to 8%-12% after seven thermal cycles with an approximate doubling in resistivity. The - 0.5 GPa average compressive stress increase in Ta thin films when cycled to 400-degrees-C is attributed to a 1.3% increase in oxygen concentration leading to a Ta unit cell expansion of 0.6%.