When it was never published in the first place.
The Royal BC Museum fish collection contains a specimen which had been locked securely in one of our type cabinets since the 1980s. It was designated as the holotype for a new species – Sebastes tsuyukii – there was even a manuscript noted on the specimen label (Westreim and Seeb 1989). It sounded legit – and no one checked until recently.
Jody Riley – my ever diligent volunteer – flagged this record when she was re-organising the fish collection. She checked what is in our old paper catalog, checked the electronic database, then looked to see if the actual specimen exists. When Jody hit Sebastes tsuyukii, and found no record of the species online, yet here in her hands was the jar with a big yellow tape label saying Holotype for Sebastes tsuyukii, she knew something was fishy.
In the end, we can take this large jar out of the cabinet designated for type specimens, Sebastes tsuyukii now is a nomen nudum (a naked name), and I can delete the species from the taxonomy in our museum database. Some database problems are easy to solve.
But this reminds me to get my fingers in gear and type the type descriptions for species I have yet to publish.