I accidentally pressed a button which moved all my assets to Starter content, but the folders they were in remained in their original location.
I moved the folders containing files back to their original location, causing some assets to move. However reference remained to some assets that were no longer in Starter Content, and some assets just stayed in Starter Content.
When I tried to move the assets that failed to move back to the original folder individually, they failed to move because apparently an asset of the same name already existed. However, in the folder there was nothing with the same name!
When you move content, the editor will create asset redirectors. Sometimes you have to manually fix up the redirectors before you can manually remove the old folders.
You can fix up redirectors by right clicking a parent folder and click fix redirectors.
It’s one of these not user friendly / just broken way of handling anything we rename or move, which the engine seems to hate. In a serious project I’d have reverted back to a commit or backup because even after successfully moving assets back / forth many references will be left broken such as pointers in the project settings or references in animation assets.