theoretically they could announce July 15 for Aug 14 Adcom I don't think that is a possibility, they never schedule any Adcoms in the first 3 weeks of August- very high level of gov. employee vacations during that time frame- not possible to get enough people together.
Examples please - that sounds so unfair to the sponsor I don't see how the FDA could justify it - that's a literal ambush and a giant violation of their Adcom guidelines. One caveat - it could happen if the sponsor received proper advanced notice from FDA but withheld the information from the public for a month or two. But is that really possible? The Adcom would (should?) show up in the FDA's meeting schedule as soon as they decide to hold one - I don't think they could get all needed attendees together on such short notice.