Create a custom SQL error log for ELMAH. Append data stored in HttpContext.Items to log entries to provide critical debugging info.

ElmahCustomSqlErrorLog.cs

using Elmah;
using System;
using System.Collections;
using System.Text;

namespace MyProject
{
    public class ElmahCustomSqlErrorLog : SqlErrorLog
    {
        public ElmahCustomSqlErrorLog(IDictionary config) : base(config) { }
        public ElmahCustomSqlErrorLog(string connectionString) : base(connectionString) { }

        public override string Log(Error error)
        {
            if (error == null)
                throw new ArgumentNullException("error");

            var webserviceLog = System.Web.HttpContext.Current.Items["CustomItem"] as StringBuilder;

            if (webserviceLog != null)
            {
                error.Detail += "\r\n\r\n-- Custom Entry Log --\r\n\r\n" + webserviceLog.ToString();
            }

            return base.Log(error);
        }
    }
}

Web.Config

  <?xml version="1.0" encoding="utf-8"?>
  <configuration>
    <elmah>
      <errorLog type="ElmahCustomSqlErrorLog, MyProject" connectionStringName="DefaultConnection" applicationName="MyProject" />
    </elmah>
  </configuration>