Spaces

Note that while using indentation of 2 spaces is allowed in general, it will cause unexpected styling in some cases, when using GitHub-Flavored Markdown.

So 4 spaces is preferred.

When 4 spaces works well

Numbered and bullets mixed.

1. A
2. B.
    - Bullet
    - Bullet
3. C
  1. A
  2. B.
    • Bullet
    • Bullet
  3. C

Numbered and code blocks mixed.

1. A
2. B
    ```sh
    $ echo 'Code block'
    ```
3. C
  1. A
  2. B 
     $ echo 'Code block'
  3. C

When 2 spaces breaks

Here using 2 spaces only and mix numbered and bullet points. The inner list is not nested.

1. A
2. B.
  - Bullet
  - Bullet
3. C
  1. A
  2. B
    • Bullet
    • Bullet
  3. C

Nesting code in a numbered list also does not work.

1. A
2. B
  ```sh
  $ echo 'Code block'
  ```
3. C
  1. A
  2. B 
      $ echo 'Code block'
  3. C

Tabs

Using tabs also works fine.

1. A
2. B.
	- Bullet
	- Bullet
  1. A
  2. B.
    • Bullet
    • Bullet