Filing an Email is buggy
This happened to me more than a few times while trying to file an email to a folder, and during the process, I accidentally hit some random keys on the keyboards, and Yahoo! Mail deleted the email which I was filing and some subsequent emails. I finally able to reproduce it more consistently by typing a number of characters and hit the backspace key once or a number of times and repeated that sequence if needed. This is awful that emails were deleted during the filing when you hit the wrong keys. As I'm typing this, I realized the problem must have occurred when I was filing an email, and I entered the incorrect folder name, and backspacing it to correct resulted in emails being deleted.