I found this out because it unexpectedly happened to me in guild.
When invited to our guild's 25 man Ulduar, the raid leader had the 10 player raid setting on and was saved to the 10 man version of it. Since I was early for our 25 man, I zoned on in to Ulduar.
Well I was not saved to a 10 man Ulduar, and naturally the warning message displayed saying you have 1 minute to leave or get saved. Unfortunately for me, no sooner did I see this message that the raid leader changed it to a 25 man raid setting. This automatically saved me to his 10 man version.
So to outline the steps of this:
Get saved to a 10 man naxx/ulduar/eoe/os
Form a raid group of 25
Keep the raid setting to 10 man
Watch whoever zones in, and change it to 25 man.
Chances are (unless they were saved to their own 10 man) that you just saved them to your 10 man version.
Have fun griefing. I did search for this and couldn't find anything. Also I never really had anything to contribute in the past that wasn't either a repost or a half-assed attempt at rep.
And if this is already in the elite section, I'd have no way of knowing.