Are there some well-known crypto APIs that leak information? Are there any well-known crypto APIs or libraries that leak or used to leak information (it can be the key or anything else)?
The classic one that I can think of off the top of my head, that also happened relatively recently and posed a pretty widespread threat, was the Heartbleed bug.  It was a big security vulnerability that would reveal authentication info in one of the most common ways communication gets encrypted over the internet, SSL/TLS.  Most computers were affected, if I recall correctly, except coincidentally Macs, because they happened to use a different library to handle SSL/TLS.