The greatest scenario is when your guild leader does it to you!
At the beginning of WotLK I was playing a paladin tank, and I was in a Heroic AN group entirely composed of guildies. The guild leader was playing a resto druid, but along the way he was collecting some tanking gear where he could, for the rationale that in the event that he had to fill in as a tank he could respec.
Essence of Gossamer drops, and he needs and wins it. Of course, he never ends up actually speccing tanking for launch raiding content ever.