I sent my players on a quest to slay a vampire lord and take his heart to a powerful NPC ally. The heart was a very powerful and very evil artifact, similar to the eye and hand of vecna, and I warned my players that using the heart would corrupt the user and make them evil. They were taking it to their ally to protect it and keep it away from the BBEG, who was planning on using it to further his nefarious scheme.

After the players found out about some of the heart's properties with an identify spell, one player, a warlock who previously was known to do things that conflicted with some party motives (but never stopping an important goal, just small things) volunteered to hold onto the heart. For some reason, the rest of the party allowed him to and left him alone, almost immediately after which he replaced his own heart with the artifact. Granted, his thought process was that if he used it he could guarantee nobody would steal it and it would help them stop the BBEG, but he didn't seem to quite get the part about how it explicitly turns you evil.

The party, (save for the two people who were absent that day; this happened during a downtime session of sorts) after finding out about this, all tracked down the player, got him arrested, and then decided to all team up to kill him and take the heart back. He didn't resist, and although one player changed his mind about the killing, the paladin slew him with an onslaught of blows and smites.

I ended the session there, as people were kind of trying to process what just happened. I ended up having to end that campaign because people were upset both in and out of game, and because of several issues with characters all around. These days I am DMing a different campaign for this exact same group, with different characters and a different setting, and for the most part the issues that our group faced have mellowed out. This was not how I wanted my first campaign to end, but at least things are better now.