I ran into the same problem recently. By digging into the system as the posters above have suggested, I found that in every case, the cause of the "failed" send was a soft bounce - with the reason being "user aborted program".
In my case, when you use the email program to execute an email send, especially if you have the regional time zone or head start option checked, and have started the program running , i.e. approved the program, any change you make to the smart list will cause the email send to "fail". The send doesn't actually "fail" as if the email doesn't get sent or delivered (such as the case of an invalid email), the send "fails" because there was a change to some part of the program after the emails have been queued for delivery. What is happening in the background (I suspect) is that the emails are lined up and ready to go, but the designated delivery time has not come. In this case, the emails are just sitting and waiting, but Marketo sees this as the email already gone out. Thus, once a change is made, Marketo thinks you are trying to send the exact same email to the same smart list that already received the original email. Remember, the system does not allow you to resend an email to someone who has already received that same email (unless of course you override this).
The only ones that will survive the process and actually get sent out are the members who had not yet been processed by Marketo before the change was made.
Two lessons learned:
1. Create your smart list as an asset outside of the email send program, then use the filter "member of smart list" in the smart list of the email send program. This way, you can make a change to the external smart list, but not the email program's smart list.
2. Once you approve the email send program - don't touch it! Unless you want the email to not send at all, i.e. you are truly aborting the entire send program, do not stop or change the email send program once it starts running.
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