every time i post, like, comment ,search up or follow a discussion ,a tab will come up and say "ERROR 404" with a duo bird that looks like he was riding his bike and fell off of it. Is this a glitch and is it happing to other people or is it just me. is it something wrong with duolingo and if it is are they even trying to fix it. I have looked on the troubleshooting disscusions and i found NADA.
Receiving a 404 error after posting a new message is a bug... the message was sent. No need to post it again, just refresh the page.
For existing discussions it means the thread was deleted as Dcarl1 said.
To be really sure, I close the tab and re-open it from my history.
(or re-open last closed tab - if your browser has that feature).
Sometimes it gets stuck reporting a 404 even though it is not true. Pressing back just takes me to the 404 again sometimes. Usually, in the case, the original 404 was just a timeout, or the local connection was dropped.