When you log error, log both error and the request url. for example
protected void Application_Error(object sender, EventArgs e)
{
Exception ex = Server.GetLastError();
Log.Error("An error occurred", ex);
Log.Error("Requested url: ", Request.RawUrl);
}
UPDATE:
if you need to avoid css files
if (!Request.RawUrl.EndsWith("css"))
{
Exception ex = Server.GetLastError();
Log.Error("An error occurred", ex);
}