Digital Extremes in their latest PC update (12.1.0) to Warframe along with regular patches and updates, also inserted a flappy birds style mini-game. In order to access the mini-game you need to own the Zephyr Warframe. Go to your Arsenal and under Zephyr Appearance's -- Click the "Z" preset that will instantly load up the mini-game. I am terrible at flappy birds....

DE_Rebeeca: https://forums.warframe.com/index.php?/topic/178607-update-1210-grinlock/?hl=update

Warframe and Zephyr honor Flappy Bird's removal from the gaming world. How? There are two ways... One, Zephyr owners should explore their Arsenal... Two, those that don't own Zephyr should think about a bird-like-word to log in to Warframe... no "@" required.

Cue people moaning that Flappy Bird marks the fall of gaming as we know it.

That's actually pretty cool. I like it when devs find ways to fit goofy easter eggs into their games like this.

The internet has confirmed that the second way of getting to the mini-game is to login with the username "flappy" and no password. As alluded to as well by DE_Rebecca.