Every WoW product springs to mind.
Starting with BC, a lot of info would be released by F&F testers anonymously. I remember one website I used to check, wowexposed, or something like that. Had all the models and zone previews before beta even came to light. WotLK had a few leaks I remember reading about, and while I honestly can't remember if this was the F&F alpha or closed beta, but the death knight mount was dubbed the "Seizure Pony" for a while as it would fall on its side and spaz on the ground. All info about it has been removed from the WoW source sites, so I assume it was alpha...was an entertaining video tho!
All in all, leaks happen.
Edit: Found the Seizure Pony - wait until 30 or seconds into it.
http://www.youtube.com/watch?v=fzGJlgbNMuc







