Well my worst mistake was in a system that we designed ourselves about 10 years ago it was in a Swords and Sorcery style but as we wanted a little unpredictability in our magic system instead of D10 to decide a result we used playing cards, 1-10 was a measure of success and face cards delivered different critical. However we decided to use Joker as a wild card in this instance the Referee would decide the result.

Well we were a long way into a campaign where our heroes were simple villagers tracking raiders who had either taken their wives and daughters as slaves, ruined their livelihoods or killed family.

The party were giving chase across the sea when i devised an attack on their ship engineered by the raiders, who had hired a mercenary to track them. i had hoped this would be a serious test to see how far my players had come and so had made this mercenary and his team very tough. However after about only 5 rounds of combat several critical on the players part and many fumbles on mine most of my guys were mortally wounded which i hate to say annoyed me. One of the PCs had received a bad wound but as they had a healer i was not worried. The healer though pulled the joker and i decided that it would be fun to give them a little exercise and ruled that a surge of power meant everyone on the ship was fully healed including the bad guys.

Guess what all of a sudden my guys couldn't miss and instead of an encounter i had intended to last 30 mins it went for the whole session of 3 hours we lost there players of a 6 man party and came within a whisker of losing the whole battle and so the entire party. I felt terrible and to compound it i said to the priest player "and so the Gods demonstrate the unpredictability of magic"

I will say that the players all said at the conclusion of the scenario that the 3 hour battle was the most thrilling experience they had role-played. I just felt bad for nearly killing a whole party and 3 very well developed players over a matter of wounded pride.

All of coheed and cambrias stuff is very RPG

Also CKY flesh into gear is very cyberpunk.

As for background music for cyberpunk games i used a soundtrack from the manga cybercity.

For modern day horror ie D20 or dresden files one day soon i like to use an album by Two Gallants called "what the toll tells"
I also like "The Man Comes Around" by Johnny Cash.