DNA polymerase can't initiate replication. But, RNA synthesizing enzymes can. So, an RNA primer is used to begin replication. This process is particularly important on the lagging strand, where each Okazaki fragment is begun with an RNA primer. Another view of this process is given here .
Many mistakes are possible in replication.
Errors can arise from modification or cross linking of nucleotide bases. Often this is the result of a change caused by chemical modification of the base or damage by X-rays, gamma rays, or UV light, all of which are energetic electromagnetic radiation. The conjugated double bonded ring structure of purines and pyrimidines is just the right dimension to absorb UV light strongly.