This is a short list of things that can easily cause bugs if you forget them.
Since widget props are encoded in embed URLs, adding props (or changing their names or types) can break existing embed URLs. Changing the name of a widget will also break URLs. You should generally only add optional props once a widget is widely shared. See ./widgets.md for more about their props.
You must add every new widget in src/widgets/index.tsx. If you forget to do that, everything will seem to work, but embed URLs wil be broken. See ./widgets.md.