I've been using a Yubikey for years, now, and on each computer I use, I install their U2F (Universal 2 Factor) bindings for the linux Pluggable Authentication Modules (PAM) support, requiring usage of my Yubikey for login and sudo access.

Today, I updated my work machine, and didn't even notice that there were new pamu2fcfg and libpam-u2f packages, updating to version 1.3.1; I never really care, as everything just works. But when I came back to my machine after lunch, I was unable to login: I'd provide my password, but my Yubikey wouldn't activate.