Anti-Fairy Goggles

Anti-Fairy Goggles were a pair of devices that were needed by Timmy Turner to see the Anti-Fairies in their first appearance, "That Old Black Magic".

Information
These "stylish" glasses, as described by Cosmo, allow the wearer to see the otherwise invisible Anti-Fairies that are causing bad luck all over the world. Timmy later uses the glasses to make sure that Anti-Cosmo isn't the real Cosmo after he was mistaken for him. Afterward, these glasses are never used or needed again to see Anti-Fairies.

Retcon
This is an example of retcon happening in the The Fairly OddParents. In all later appearances by the Anti-Fairies, these glasses aren't needed to see the Anti-Fairies at all, and there is absolutely no mention of the Anti-Fairies being invisible whatsoever. A later appearance by the Anti-Fairies in a Jimmy Timmy Power Hour special had Jorgen explain that this was simply wished away in an earlier appearance by them, but this never happened either.