In "The Last Train to Oblivion", the ghost of Casey Jones was able to pass on after managing to prevent a train crash. Except...there was no danger of a train crash until Casey kidnapped Peter and started the stolen train down the tracks in the first place.
It wasn't that he had to avert a crash. It was that he had to perform his legendary, spectacular run, pushing the engine beyond the limit, doing what was thought to be impossible... AND not crash. It wasn't penance, it was post-mortem OCD.
In "No One Comes to Lupusville" the boys visit a town full of vampires and werewolves. Three dozen episodes later they're hired by a Boldavian count who claims to be the last vampire, and no one gainsays him. Guess we know who ultimately won the battle in Lupusville ...
To be fair, we do see more vampires later in the series. So the count is basically just being emo.
Or he's just the last vampire in Boldavia.
In the first episode, "Ghosts R Us", the Class 10 runs over a bunch of cars using its bike. What's disturbing is that we only see one car have its passengers safely get out before getting destroyed. Did this entity just kill dozens of innocent people before it was destroyed?
JMS explicitly notes this on the commentary for that episode, saying that the network never let that happen again.
In "The Cabinet of Calamari" when the disappearing victim escapes from the cabinet, he asks for some money to call his wife. Winston informs him that since he's been gone, the cost of payphones has increased. The man was in there for years without ageing or noticing the passage of time, and chances are his wife has aged or moved on without him.
In "Buster the Ghost," the titular ghost decides to kill himself after yet another failure at a job, this time as a tooth fairy. As he says this, he makes a Finger Gun gesture pointing at his head. As shown in other episodes, ghosts can shot with their hands in that gesture. It may have been possible that he really was going to commit suicide within a few seconds if he hadn't been interrupted by his talking watch.