Whenever you switch to another account while the Skins menu is open, the "Successfully updated your skin" message shows up in the top-right corner of the window.
It should be noted that this happened to me while switching from one account that has access to the Mojira cape to another account that does not. It happened both ways. I'm not sure whether this is relevant.
- Be logged into at least two accounts in the launcher
- Open the skins menu
- Change to another account using the top-left menu while in the skin menu
→ The message will appear