Yahoo's unlimited storage email service fails under certain conditions that prevent users from accessing their mail. Most likely, poor testing is to blame.
According to The Wall Street Journal, when users leave too many messages directly in their inbox, login becomes impossible:
Whenever we tried to log on, we got a “LaunchCascadeError-ymws:Server.MailboxOpenFailed” error message.
Turns out Yahoo isn’t really prepared for users doing what we do–namely keeping all their mail in a single inbox–as opposed to moving them into sub-folders. (We don’t spend any time sorting because doing so defeats the whole purpose of having a single back-up email system that doesn’t require any thought or tending.) When the number of emails in a single folder gets too big, a Yahoo Mail account crashes and burns.
Here's how Yahoo describes its unlimited storage email service:
Unlimited storage gives normal email account users like you an opportunity to not have to worry about hitting a storage limit. Basically, the idea is that now you can save your correspondence and memories and never worry about deleting older messages to make room for more.
THE IT PROJECT FAILURES ANALYSIS
I suspect the development and QA folks didn't think the product would be used as described by the Journal and therefore didn't test for that situation. Yahoo should have conducted boundary testing specifically to see what happens when a user's inbox is loaded with lots of messages.
In all programming languages, bugs most often appear at boundary conditions. Thus, you should choose test data in which values lie along data extremes. Boundary values include maximum, minimum, and just inside/outside boundaries.
Yahoo responded to the Journal saying: "You’re using the product exactly how we want it used.” Obviously, PR and development don't talk enough.