Make a Google Sheets dropdown that catches inconsistent entries

Suppose a tracking sheet has New, In progress, and Done as dropdown choices, but someone needs to enter Waiting. Decide how that entry should be handled:

• Keep the default rejection if every row must use one of the listed statuses. Sheets will reject Waiting.
• Choose Show a warning if Waiting is an occasional exception worth recording. Sheets allows the entry with a warning, but Waiting stays off the dropdown list.
• Add Waiting to the dropdown items if everyone should be able to select it.

Set up the dropdown

On a computer, select the status cells in Google Sheets and go to Data > Data validation > Add rule. Under Criteria, choose Dropdown, enter a status, and use Add another item for the remaining choices.

To allow unlisted entries, open Advanced options and select Show a warning under If the data is invalid:. Otherwise, leave the default rejection setting in place. Click Done.

To change the choices later, select the cells, open Data > Data validation, and edit the items under Criteria.

Source: Google Docs Editors Help. These are documented instructions; no live Sheets test was performed.