When M-S-RET is one key press too much: M-RET to the rescue!
While M-S-RET is totally fine to insert list items with a checkbox I find myself often in a situation where I press M-RET and only afterwards recognize that I forgot the S part. Then I have to delete the new line and insert a new one this time really using M-S-RET.
Below advice helps me getting new list items with checkboxes even when only using M-RET to insert a new one.