"I'll spell it out clearer... you have PROOF that it is the replication system itself that allows for "errors" to creep in with no external mutagenic factors?

"Intrinsic DNA damage checkpoint

Another interesting aspect is the possible existence of an intrinsic DNA damage signal in a normal cell cycle, in the absence of external cues. Indeed, the replication process by itself can be genotoxic. Replication errors occur stochastically during nucleotide incorporation, and structural intermediates normally arising during unperturbed DNA replication, such as unwound DNA and single-stranded regions, are more fragile than double-stranded DNA organized in a chromatin structure. In addition, single-strand and double-strand breaks are generated by the nicking–closing activity of DNA topoisomerases, which are required to remove torsional stress ahead of the replication forks."

http://www.nature.com/emboj/journal/v17/n19/full/7591246a.html