Secrets leakage is a growing problem affecting companies of all sizes, including GitHub. They recently made an announcement on their blog regarding an SSH private key exposure:
[Last week, GitHub] discovered that GitHub.com’s RSA SSH private key was briefly exposed in a public GitHub repository.
The company reassured the public explaining that the key was only used to secure "Git operations over SSH using RSA," meaning that no internal systems, customer data, or secure TLS connections were at risk. They reacted immediately by detecting the incident and changing the key: