I've seen this a couple time and have found help elsewhere, but thought I should add it here too. If you're messing with the header on an InfoPath form and end up not being able to open it due to an error with a header tag in your .xsf file, it's fairly easy to fix. Kinda.
- First rename the .xsn to a .cab.
- Use WinZip or what ever archive program you want and extract the contents of the .cab.
- There you'll see the Manifest.xsf. Open it in Notepad or other text editor and find your offending <header> tag and delete it.
- Save the file (make sure it keeps the .xsf extension).
- Now you'll want to zip these back up into a cab file again. If your archive program supports this, use it. If not, try iexpress.exe (Start -> Run -> iexpress) which is likely already on your machine (new to me). It'll walk you through creating a cab file from the files you previously extracted.
- Finally, rename the .cab back to a .xsn and you're good to go.