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Ultradense domain walls are increasingly important for many devices but their microscopic properties are so far not fully understood. Here we use molecular dynamic simulations to study the domain wall stability in the prototypical ferroelectric BaTiO3 combining core-shell pair potentials and a coarse-grained effective Hamiltonian. We transfer the discussion of the field-driven nucleation and motion of domain walls to thermally induced modifications of the wall without an external driving force. Our simulations show that domain wall dynamics and stability depend crucially on microscopic thermal fluctuations. Enhanced fluctuations at domain walls may result in the formation of critical nuclei for the permanent shift of the domain wall. If two domain walls are close—put in other words, when domains are small—thermal fluctuations can be sufficient to bring domain walls into contact and lead to the annihilation of small domains. This is even true well below the Curie temperature and when domain walls are initially as far apart as six unit cells. Such small domains are, thus, not stable and limit the maximum achievable domain wall density in nanoelectronic devices.