How to simulate bad backup file with 'ultraedit'
You don't need UltrEdit to destroy a backup file.
Actually, this really powerful text editor will allow you to nose around inside a backup file and even save it without rendering the file completely unusable ... 

Not that you should, obviously. 

Good old Notepad is far more capable of causing this sort of damage. 

Just open [a copy of] a Backup file and save it again.  Mischief Managed. 

Notepad's lack of acknowledgement of Operating Systems other than Windows - and the line endings that they use - will cause it to rewrite the file with extra characters that make its contents complete nonsense to Sql Server.