I found out by accident that if you attempt to start "The Ghost of Old Hroldan" when the Dark Brotherhood questline is primed to start, the kidnapping sequence will interrupt Ghost of Old Hroldan's start up. This doesn't break the quest, far as I can tell, but since the quest hinges on you being awoken by the inkeeper's scream, the fact that the kidnappng can happen from Old Hroldan's bed when Ghost of Old Hroldan is being started up feels like an oversight. (The fact that these happen to be the only two quests in the vanilla game that require you to sleep is likely why this went unnoticed for so long)