SPListItem - SystemUpdate, UpdateOverwriteVersion, Update

If you have ever tried to figure out the various SPListItem Update methods the table below, stolen from this blog, gives you an excellent overview:

Method	bSystem	bPreserveItemVersion	bNoVersion
SystemUpdate()	true	false	false
SystemUpdate(false)	true	true	false
UpdateOverwriteVersion()	false	false	true
Update()	false	false	false

SPRequest object was not disposed before the end of this thread

Analysing SharePoint logfiles you might find a lot of Potentially excessive number of SPRequest objects warnings in the logfile.

In order to figure out the reason behind this warning you can add a registry key thereby getting a full stack trace as outlined in this blog.