...the harder they fall.
The first round of any Swiss tournament matches the top half of the field against the bottom and, as a result, there are many unequal pairings. If chess ratings were set in stone the result should really be 1-0 all the way down to the midpoint but it doesn't usually happen that way and we expect a few draws and an upset or two where an up-and-coming David manages to bounce his pebble off the cranium of some modern Goliath.
The same thing can happen in a chess.com tournament when, by necessity, there is a wide range of experience. The trick is to play carefully and confidently, because Goliath makes his share of mistakes.
The games I've selected here are taken from the “Biggest Upset” posted in my own recent chess.com tournaments. Generally speaking, they aren't shown for their excellence but because a lower rated player had a satisfying win.
In the first game there were 430 rating points between bjmin and his higher-rated opponent, and the game went pretty much as you'd expect. Mr No-Name was having a lovely time gobbling up pawns and exposing the White king. If he'd taken a moment to think before snaffling a fourth pawn he'd have won easily. But “ifs” don't cut it on the chess board and full marks go to bjmin for hanging on in a lost game and seizing his opportunity when it came.
The second game is one that angelor, rated 1474, won against N.N. on 1858—and that's a difference of 384 points. Angelor showed right from the outset that he was willing to get in and mix it with his stronger opponent and, if his 7...Qh4 threatening mate was easy enough to counter, it created weaknesses in White's position that Angelor was later able to exploit. His willingness to castle long, right under the nose of the marauding queen, left him in control of the open d-file; White's attempts to shore up the king-side weaknesses converted this to a permanent advantage. By the time White had played 24.Bg5 attacking the d8 rook there was blood on the board and none of it belonged to Black. A very nice exploitation of open lines.
Here's an upset of Biblical proportions where BexterDogg, rated 1293 defeated an opponent on 2285—near enough to 1,000 points difference. It's easy enough to see what happened. NN intended to play 10.Nf3 winning a piece for the knight protects mate and attacks the bishop while discovering an attack on the queen by the c1 bishop. If the queen moves to a square that defends the bishop she is still unable to recapture because if 11.Nxe5 Qxe5 12.Re1 pins the queen. We've all done it—moved the wrong piece, hit the Submit button, and recognised the blunder a heartbeat too late. All we can do then is hope that our opponent suffers an attack of chess blindness or neuronepaenia and fails to capitalise on our misfortune. It's not often we get that lucky.