The UK Ministry of Defence released classified documents about its nuclear submarines. The documents were requested under the British Freedom of Information Act. Any still-classified information in the documents was supposed to be redacted, except that the government official in charge of releasing the documents didn't grasp that changing the background color to black does not actually remove any of the redacted text.
Anyone could read the classified information by copying and pasting the black-on-black text into another document.
Search engines that scanned the documents, such as Google, also had no problem reading, indexing, and caching the documents. Search engines don't care what you use for a background color.