In that case, race conditions could cause file corruption, duplication, or worst case… well unknown issues like this one. In general, we don’t encourage using two sync providers to sync the same set of files across the same set of devices. Hmmmm is there any chance you configured the Obsidian vaults to be in a parent folder on one of your devices when using Obsidian Sync, and then when Obsidian Sync tried to replicate the folder structure, Dropbox picked it up and synced it across, causing infinite recursion?