Controller or battery missing or unresponsive after teleport
Sims are required to complete avatar transfers to the new region within a certain timeframe. If either the source or destination sim is particularly crowded, or the avatar has many attached scripts, then attachments can sometimes get dropped during the process. Typically, your viewer will not be informed of this, causing the attachment to still be visible on your screen, even though it is impossible to detach, has no accessible contents, and no one else can see it. This used to happen most frequently with sculpted hair that had many resizer scripts (one per linked part) inside.

This gets aggravated when multiple attachments share one attachment point, and the controller's default position on the avatar's chest often causes it to conflict with shirts and other clothing. Occasionally, the battery will go missing in the same way if something else is attached to the spine. Fortunately, most modern mesh clothing can be relocated without difficulty, as rigged mesh fits to the avatar's body regardless of its root attachment point. Whenever possible, avoid multiple attachments on the same point. If you find there is some other conflict between your attachments, try relocating them as well.

Solution: If you're using Firestorm, see the menu item Avatar > Avatar Health > Refresh Attachments. Otherwise, close your viewer and log back in; you will need to re-attach your controller or battery afterward. Make sure no other attachments are on the same attachment point as your main controller. (Tip: rigged/fitted mesh can be attached anywhere.)