My Pebble flashes on and off when charging

This can occur when the battery is completely drained.
The watch tries to boot, which disables charging, and a loop occurs.

To fix:

• While connected to the charging cable, press and hold any button on the watch, until an error code appears on the screen.
• Release the button. Now, let the watch charge for some time (at least an hour).
• Finally, press a button to exit the error code.