Yes: 1 game per 2 million
https://wismuth.com/elo/calculator.html#rating1=1000&rating2=2400
can a 1000 rated player defeat an IM
#7
I once witnessed a case where an old lady drew against an IM in a swiss open tournament. The old lady made odd and bad moves, the IM took it lightly and blundered. In the end the IM could barely draw.
3+ check does not count
Yes it does. The OP did not restrict the type of game played.

It's not possible for an actual 1000 Elo player to beat an IM in a normal rated otb game, but it is always possible for an IM to take such a game too lightly and make a devastating blunder while not paying attention. Although the IM might well be able to pull out the win anyway in such a situation, I have seen players in such a situation "blow up", resign and go off in a huff cursing their stupidity.
On chess.com it's possible that a strong player neglected to choose an appropriate beginning rating, so such a player with the default 1200 rating could actually be strong enough to beat an IM.

There's less than 1% chance if they play their average in theory.
But in practice the 1000 rated player might be underrated. Happens all the time. I was 1400 6 months ago and I would beat 2100s from time to time but that's because I was underrated.
Ask yourself the question: How does a 1000 rated player get paired with the IM? Probably because he's doing very well in the tournament
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