The theory for recording of thermally blocked remanences predicts a quasilinear relationship between low fields like the Earth's in which rocks cool and acquire a magnetization. This serves as the foundation for estimating ancient magnetic field strengths. Addressing long‐standing questions concerning Earth's magnetic field requires a global paleointensity data set, but recovering the ancient field strength is complicated because the theory only pertains to uniformly magnetized particles. A key requirement of a paleointensity experiment is that a magnetization blocked at a given temperature should be unblocked by zero‐field reheating to the same temperature. However, failure of this requirement occurs frequently and the causes and consequences of failure are understood incompletely. Recent experiments demonstrate that the remanence in many samples typical of those used in paleointensity experiments is unstable, exhibiting an “aging” effect in which the (un)blocking temperature spectra can change over only a few years resulting in nonideal experimental behavior. While a fresh remanence may conform to the requirement of equality of blocking and unblocking temperatures, aged remanences may not. Blocking temperature spectra can be unstable (fragile), which precludes reproduction of the conditions under which the original magnetization was acquired. This limits our ability to acquire accurate and precise ancient magnetic field strength estimates because differences between known and estimated fields can be significant for individual specimens, with a low field bias. Fragility of unblocking temperature spectra may be related to grain sizes with lower energy barriers and may be detected by features observed in first‐order reversal curves.