My friend once accidentally turned the difficulty to Peaceful after spawning the Wither. He then went back to the Nether to get more skulls

...whilst forgetting that it was still on peaceful?

No he switched it back to easy. I think it was on hard when he summoned the Wither and wanted to switch it for an easier fight. Also since I play on console this was on Bedrock so you can probably understand why he changed the difficulty

Oh I see

They get the Spooky Scary Skeleton achivement