can a 900 beat a 1800?

It's possible. Though the possibility is not high, but surely possible. Against the true 1800s.

Because players are human, you can't be sure about the ratings.

thatis true

I did beat my coach with 1850 rating when i was around 1000 sooo its possible now im going to play ranked official chess

4 games in 10,000
https://wismuth.com/elo/calculator.html#rating1=900&rating2=1800 

Yes.

Never.

My daily rating is 750. I'm about to beat a 1850 in a 48 hour game. I can't mess this up!! I'm +1 right now in end game.

my main acc is 1800 and my fide is 1800 but the lowest i can lose to is 1100 so no I dont think a 900 could ever beat me unless in a fast time zone. the highest I beat is 2300

https://www.youtube.com/watch?v=GlpE-K6QrUE

Yes, if the 1800 doesn't work hard and take it serious.

it is absolutely impossible

well i recently beat 600 plus player and iam a 100 player -- maybe it was just luck i don't know but if that is possible then a 800 against a 1600 is possible don't you think

I am sure this is something normaly that a 900 beats a 1800, I mean nobody is perfect and I think when a 1800 is playing against a 900 then he is like "Oh, that will be easy", them is easier to make a mistake. Of course, I am only 700+ so my opinion is't right