When you delete a Snap, we will set it to delete from our servers. We will also make our best attempt to remove it from your friends' devices. There are a few situations where this might not work. For example, if someone has a bad internet connection, or is running an old version of Snapchat, the deletion may not go through. Also, if someone exports the Snap to their Camera Roll, the sender cannot delete it from there.
If you post a Snap to your Story, one of your friends comments on the story so that it is in the Chat screen, then saves that Chat, that Snap will remain in the Chat screen as well.
Learn more about when other kinds of content are deleted from Snapchat servers.