This happens when you inadvertently make someone a member of a program in a campaign. Often, you'll see this step in a smart campaign:
Change Program Status=Sent
but this occurs before the Send, or even after. But Marketo will mark the Status regardless of emailability even though it doesn't actually send the email. To fix this, block bad emails at the Smart List level. You could also do a choice Step like
if Was Sent Email=X, then Change Program Status=Sent
Another way that I see often is you'll upload people to a Program, which automatically places them in the First Program Status (usually=Sent) instead of a Status like "Uploaded" and then Sent is a later Status. To fix that, you'll need to adjust your Channel Statuses in Admin.