SharePoint and thread safety

var list = web.List["MyList"]
list.Items[0]["Field1"] = "foo"
list.Items[0]["Field2"] = "bar"
list.Items[0].Update() // nothing is updated!

SPListItem item = list.Items[0]
item["Field1"] = "foo"
item["Field2"] = "bar"
item.Update() // updated!

SPSite siteCol1 = new SPSite("http://localhost");

SPWeb web11 = siteCol1.OpenWeb();
SPWeb web12 = siteCol1.OpenWeb();

web12.Title = "web12";
web12.Update();

web11.Title = "web11";
web11.Update();