Quack Overflow is Stack Exchange's 2018 April fools joke. It consists of a little rubber duck in the right lower corner of Stack Exchange sites. It's accompanied by a text bubble which says "Can I help?":

Upon clicking on the duck, the user gets lead through a short rubber duck debugging session, which ends with a "Quack" by the duck.

The Rubber Duck Debugging session

If the user clicks on the duck, they get asked whether they have a microphone. The duck will then instruct the user to explain their problem out loud, regardless of the previous answer. It will then try to analyse the spoken language and tell the user to speak up. Afterwards, it will think about an answer, type it, and finally end with a "Quack", which is both shown and played over speaker.

Afterwards, it tells the user about rubber ducking. The user can then decide whether to tell the duck another problem, or state "I hate this duck". After a final goodbye, the duck will be gone (for that site).

The microphone is never used throughout the session.

Timeline

Quack Overflow's rubber duck appeared on April 1st 00:00 UTC+14:00. It's shown on non-localized Stack Exchange sites, including Meta Stack Exchange, but not on per-site metas.