1) If you open a new submission from a particular user you watch, in another tab and and delete the submission while all the submission entries (in case of multiple submission entries) haven't been retrieved in that new tab, then all those non-retrieved entries will be forgotten by the system once and for all.
2) Even if all the entries are loaded when you delete the collection from your deviantwatch list, and by mistake close the tab containing the entries, then if you didn't memorize the deviant user that made the submission, you'll be in major problems (since the system has deleted any notion of that submission when you deleted it from your d-watch "messages" tab).
So my question to DA.com would be: why the heck is there no "mark as viewed" button instead of (or if so desired, in addition to) the "delete" button, upon whose clicking that submission becomes inaccessible from the "messages" tab and instead becomes accessible from a "viewed messages" tab ? Countless times I have lost (and now again, before I started writing this) new submissions due to this nonsense design of the deviantwatch system. To summarize: I want to clean up my "messages" tab in the new deviations list, but I don't want to "delete" those submissions, and instead I want to access them from, say, a "viewed messages" tab.