[rubicon33]

'Every time a cell divides there is some chance of a genetic error occurring.'

Why is that cell allowed to exist then? Isn't there a quality control mechanism or something, that can detect an error occurred and delete the cell?

[epistasis]

There are lots of DNA repair mechanisms, as well as checkpoint mechanisms that prevent damaged cells from dividing further.

However, as with anything digital, there is some error rate still. The most common type of cancer mutation will deactivate, p53, a protein that is one of the checkpoint genes.

Having an error rate is essential for evolution and variety, so its not entirely a bad thing.

Wikipedia's entry is a pretty good entry point:

https://en.wikipedia.org/wiki/DNA_repair