Abstract <p>We discuss 13 known rapid increases in radiocarbon content in the Earth’s atmosphere and all proposed to date hypotheses of their origin. Comparing the occurrence probability of 4 most powerful events, prescribed likely to the solar superflares, with the Kepler’s mission data it results in a significant discrepancy at least assuming flare energy <i>E</i> ∼ 10³⁵ erg. The hypothesis of solar superflares (or sequence of flares) cannot likely be the only one accounting for the origin of all measured ¹⁴C signals. For example, some events could be explained by the Solar system encounter with the interstellar inhomogeneities (clouds). For two of them (660 BCE and/or 813 BCE) there is a critical experiment on ³He content increase in Antarctic ice supporting this suggestion. However, none of the proposed hypotheses can reliably satisfy the experimental data.</p>