This annoyed me - looking for explanation

So very correct.

Having been a teacher, instructor and tutor for over 40 years, I have witnessed what cheaters are all about.

Unfortunately, nearly all societies emphasize the symbols of achievements (grades, ratings, trophies) over self respect and honor, and fail to equip their charges with the tools which enable them to achieve honorably.

Those who would falsely accuse others of cheating are cheaters themselves, for lying about failure is hardy different from lying about success.

Yes, it is a chess.com game - and not a Live game!

I have since been in contact with my opponent and he has apologised for the accusation, but does seem to think cheating is rampant on chess.com.

We all have bad days...

As for cheating, well, it is rampant for higher levels (say >2300 CC, >1900 live chess).

thats a laugh how could you be cheating especially as whote had the better position in my opinion at the start

I don't think the presence of cheaters online matters all that much, and here is why.

Most cheaters will likely us the same program (at the same rating) all the time.  For the other player (assuming s/he is rated <~2000), there is virtually no difference between playing a computer and playing a person of the same level.  As long as the cheaters rating is representative of the strength of the program they are using (i.e. they are not playing "for real" a significant portion of the time), then it's as if you are simply playing a "better" person.

Another way to look at it is to suppose that the cheater, rather than use a computer, is using a person.  Suppose he's getting moves from his friend who is rated 2200.  In effect, you are playing the cheater's friend, but just under a different screen name.

The bottom line is that a cheater can not distort your rating unless he is only cheating on rare occasions (e.g. he is rated 1300, but on his match with you he uses a program rated 2500).  In those cases, you would get a loss that would push your rating down lower than it should.  But I think this is probably the rare situation.

 It matters in online tournaments.

Only if the player does not use a program outside of tournaments (and thus has a lower rating) and/or there is cash or other prizes of value on the line.  I was thinking mostly about friendly/casual play when I wrote that, but it should extend to any other format as long as the cheater cheats consistantly.

Many tournaments for top rated players are categorized as >2200 or >2300 and they do not have a upper rating limit. So the genuine players above 2200(or 2300) will be paired against a 2900 rated cheater and end up losing the tournaments.

The same would apply to a lower rated tournament, where a 1401 player is matched up against a 1599 cheater, though the real player would have a better shot at beating the cheater in that scenario.

I guess even if there are no cash or other prizes, the "status" of winning a tournament is enough of a reward for some/most to make this a concern.  When I wrote my first post I was thinking mostly about the effect of cheating on real players' ratings, and I stick to my hypothesis that it doesn't matter (in the long run).

Seriously, do it.

which means "it matters in the short run, esp in tournaments". And that is my hypothesis.

The answer to your question is that computers are known to have the ability to make their opponents play moves like your opponent's 21. Ng5?? (turning a winning game into a losing game) and then fail to take advantage of them by playing 21. ... Ng2! [I can see how both of you missed this delightful tactic, where white has three moves but either loses a rook with check, or loses his queen to a fork assisted by a pin]

But, seriously, the answer to your question lies firmly in the ego of your opponent, which needed to be protected from a traumatic experience.

By "long run" what I really mean is "after the cheater has played enough games that his cheating rating is representative of the program's strength."  I you play (outside of a tournament) against a cheater using a 2500 rated program, it won't materially affect your rating unless the cheaters actual rating far away from 2500.

oh! very disgusting...how can anyone accused a good player as a cheater??.he/she don't know how to play chess..just teach him the right manner when playing chess...

One opponent accused me of being a cheater in an online game recently. He told me that he checks all of his games on a computer after the tournement is finished and would report me if i met a certain threshhold. I told him what he could do with his computer; he ended up blocking me. I can't imagine why.