Every single expansion without exception. Most only leaked shortly before the announcement, once all kinds of people outside of Blizzard were involved in setting up the stage and such.
WotLK and Cataclysm leaked in detail, they were featured in this very site. For Cataclysm, the whole alpha client leaked, so people datamined the shit out of it, too.
For MoP, the trademark appeared before anything else, which led to many fake leaks that got many things right, but I believe one with correct information appeared a week or so before Blizzcon.
WoD had its own thread with some outdated information a couple of weeks before the announcement. Probably from someone who was only involved early in development, before a bunch of stuff was cut or changed.
In Legion, a dude popped up in the expansion speculation megathread and in two posts gave a fairly small but accurate description the day before the announcement. People called him names and moved on without giving it much thought.
Something similar happened with Shadowlands, and it included a picture from the cinematic, Bolvar with blue eyes. That one got more attention, since people went back and forth trying to debunk or confirm the picture.
I don't exactly remember how BfA leaked, but I do remember watching the announcement and thinking "yep, just what I saw earlier".
Some links:
https://www.mmo-champion.com/threads...K-Massive-leak
https://www.mmo-champion.com/threads...f-Draenor-LEAK
https://www.mmo-champion.com/threads...or-Leak-Inside
https://www.mmo-champion.com/threads...2#post35267612