I'm not sure if this qualifies as a bug per se as during "normal" gameplay this shouldn't really happen, but...
I was playing Adventure mode and was on a quest to kill a titan. I killed the titan, did a few more quests, and then my roommate decided to vacuum while running the microwave, blowing a fuse and cutting power to the apartment, and of course, turning off the computer in the process.
The next time I started the game I was yet again on the titan quest, since I hadn't saved my game since. However, going into the cave where said Titan lived, I found that it was already dead in the same spot where I killed him before! As a result the Titan quest is "unfinishable" seeing as how he's already dead. My guess is that this will probably mess with the Legends as well. The other quest areas I had been to since the blackout seemed to be similar, but since they all came from the same questgiver that gave me the titan quest, I couldn't try getting more quests with him to see if he would send me out to kill an already-dead monster.
While I have no idea how the code works and may be talking out my ass here, my best guess is that areas get saved to the region when you leave them, but the character file doesn't, hence the disparity. Perhaps setting the character to auto-save when you leave an area would fix this issue?
That said, it shouldn't be a major problem in general unless you have someone routinely cutting the power to their computer during play.