If a spammer is deleted by somebody else while you are looking at the review queue, you cannot tell. Trying to delete them subsequently gives a confusing error message, and the user is still apparently in the queue.
The sequence was approximately:
- spammers gonna spam
- spammer is flagged and put in the queue
- I opened the queue page
- somebody else deleted the spammer
- I hit the button to delete the spammer
- the error message is displayed
- but the spammer still appears on the page
- open a bug on Meta
This bug – Error on Delete User in pending-approvals list – seems similar.