Recent ab initio molecular dynamics simulations have shown that radioactive carbon does not normally fragment DNA bases when it decays. Motivated by this finding, density functional theory and Bader analysis have been used to quantify the effect of C --> N transmutation on hydrogen bonding in DNA base pairs. We find that C-14 decay has the potential to significantly alter hydrogen bonds in a variety of ways including direct proton shuttling (thymine and cytosine), thermally activated proton shuttling (guanine), and hydrogen bond breaking (cytosine). Transmutation substantially modifies both the absolute and relative strengths of the hydrogen bonding pattern, and in two instances (adenine and cytosine), the density at the critical point indicates development of mild covalent character. Since hydrogen bonding is an important component of Watson-Crick pairing, these C-14-induced modifications, while infrequent, may trigger errors in DNA transcription and replication.