Purging unwanted linetype bloat
I have been recently running across AutoCAD DWG files that contain hundreds of non-purgable linetypes, some with strange names, some that appear to be nested in mysterious blocks, etc. With help from the Autodesk Forums and some independent research, I became pretty sure these were created by importing Microstation DGN files.
Working on one of these drawings, I was able to use SuperPurge to "hard" purge these linetypes, which in turn released the reference on hundreds of anonymous blocks, some dictionary entries, etc. What a mess. Upon further research, I found this (albeit 6 months old) article from Kean Walmsley titled Purging unwanted DGN linestyle data from an AutoCAD drawing using .NET which goes into detail about how and why this mess gets created.
Then as if magic, yesterday Autodesk released a "hotfix tool" of sorts yesterday to specifically clean up this bloat (be sure to read the readme file for installation instructions). I ran this tool on several infected drawings and the mysterious linetypes and other bloat are deleted. It also releases the reference to many anonymous blocks, those can be purged with the built in Purge command.
I love the fact that Autodesk released this tool, but I kind of wish it was a standalone .EXE that could operate on multiple drawings. Nevertheless, I hope this gets incorporated into the normal Purge command in a future release. Thanks Autodesk!
EDIT (June 25, 2013): Jimmy over at JTB World has a post on this, and apparently discovered that this tool could lead to data loss under certain circumstances. The download has been removed from the Autodesk website for now. According to comments here and on JTB World, Kean is working on an update and it will be released soon.
EDIT (July 30, 2013):
An updated HotFix was released on July 23, 2013, you can find it here: http://usa.autodesk.com/getdoc/id=DL22002791
EDIT (March 28, 2014):
AutoCAD 2015 and AutoCAD LT 2015 both include the ability to purge DGN linetype bloat. It's now built into the regular Purge command. See this post for more information.