The former, though likely still at a pretty low rate. These instabilities can result in mis-pairing at replication which results in a mutation in the replicated

The former, though likely still at a pretty low rate. These instabilities can result in mis-pairing at replication which results in a mutation in the replicated strand. Article suggests that this may be a significant source of mutations that was previously not considered. Article says: We find it to be several orders of magnitude greater than the observed rate (10−8 per base pair) of spontaneous mutations through, for example, copying errors, suggesting that tautomerisation may well play an important role in point mutations.