Yes, indeed, R didn't throw any errors. Instead, R expanded the list so that it contains 10 members in total. It modified the 10th member to contain the value you passed in. All other members between the 4th and 10th members were populated with a value of NULL because we did not define any members for those positions.

Remember: when adding new members using an index inside the double-bracket operator, you should always use an index of length(your_list) + 1. That way, you won't have any members populated with NULLs.