Happy Eyeballs
The algorithm that makes a dual-stack client try IPv6 and IPv4 in parallel instead of hanging for a minute when the IPv6 path is broken.
The problem defined the transition years: a machine would get an IPv6 address that looked functional but whose route out was broken, and since IPv6 is preferred the browser waited out the full TCP timeout before trying IPv4. Users concluded that IPv6 makes the internet slow, which held up deployment for years. RFC 6555 from 2012, updated as RFC 8305 in 2017, says the client should start the IPv6 connection first and then start IPv4 after roughly 250 milliseconds if nothing has happened. Whichever finishes first wins and the other is aborted. The current specification extends the same race to name resolution.