UPDATE
For the benefit of those wondering what's going on with
@Sam2015's case, after reviewing some of the documents USCIS included with his package, we've been able to determine that the package got rejected by USCIS Chicago Lockbox due to early filing. Looks like the package was received by one of those over-zealous 'this is my job, and I know exactly what I'm doing' reps (who actually isn't so smart after all) and is not aware or simply chose to ignore the early filing memo directive, hence decided to reject the petition as filed one month early. Sam will therefore wait till July 1st to re-file.
While it is highly desirable to not experience any hiccup during the DV AOS process, getting rejected by the Chicago Lockbox is actually not as ominous as it seems, package gets returned within 3 weeks or so of filing, check isn't cashed, and the case can be re-filed almost right away. If the case makes its way all the way to the FO, and the IO decides to apply the 'inability to establish priority date as at when the case got filed' clause, this will be a case denial (not a rejection as in the case of the Chicago Lockbox), the enclosed check would have been cashed and is none refundable, couple or several months would have gone by before the denial notice is issued. Of course, if this happens early enough, the petitioner can equally re-file, only a new AOS fee would have to be made.