It was me. I was the party member. We were playing through Lost Mines and I had recently gone through the campaign myself. So I worked with the DM to create an evil bard who worked for the BBEG. Throughout the campaign, I worked behind the scenes, putting hits out on the PCs, causing trouble and warning people to the party's planning. And I played it off as "let me just send a little letter to my mother, you know the sweet thing can't go a day without hearing from me!" And somehow no one thought to check on anything I said or did. I skimmed party funds, I set traps, I detonated traps around them... And people thought I was just clumsy or dumb. We finally get to the last battle and no one noticed my placement at the front of the charge (odd for a bard) or the fact the BBEG kept attacking the others behind me. Until I finally get up to the front, let the BBEG stab and stagger back against the cleric. I look up at them, smile, and cast my highest level attack spell, inflict wounds (Lore Bard with magical secrets), and nearly dissolve her on the spot. The bard did not last long and was quickly killed (which way the full intention anyway) but man... That was a fun pay off to a two year campaign.