One of our products is a messaging processor which uses XMl internally. Somehow someone messed up and managed to insert characters into the XML which are not valid, making the message "invalid XML". By the way "Invalid XML" is a contradictio in terminis. I would rather refer to "invalid XML file" as A Text File That Looks A Lot Like XML But Isn't.
Solving the issue would require updating the shared core component which is used by many applications. Updating the core component was not an option as a bugfix.
We decided to fix the bug in the following major release, but still needed a workaround for the problems in the field.
Enter Html Agility Pack. It is a tolerant library that can read HTML files (text files looking a lot like SGML or like XML) and convert them to XHTML. We inserted a processor into the processing chain that XMLlifies the message as it passes the processor, et voilà!