There is more to unforced errors than 'serious collisions', and what is to say a serious collision is 'unforced'.... two cars battling can lead to a bit of contact- see Alonso on Vettel at Spa this year as an example. They all make errors, some more memorable, or more spoken about on here in any case.
As for Monaco '04, that was the other Williams, i.e Ralf, not JPM, and was really Alonso's own error for running out on the marbles through the bend of the tunnel. From that race Montoya can probably attest to a guy in a Ferrari jumping on the anchors in the tunnel and squeezing him against the barriers as he took avoiding action....
Monaco 04 was simply Jarno Trulli's day. I don't think Fred would have won that anyway.
As for the unforced errors, Fred is probablty the best at not making those errors.