why does this say a close game lost by a mistake? what was his mistake? resigning?

why does this say a close game lost by a mistake? what was my white opponent's mistake? resigning?

https://www.chess.com/analysis/game/live/19752728759?tab=analysis

The game felt fairly even to me.. But the side bar analysis shows white(my opponent), ahead.

When I analyse it says " One player was winning, but then gave it away."

I wouldn't pay too much attention to that blurb.

I would say it was a mistake to resign though. There was plenty to play for. Good for you though!

usually the blurb isnt completely wrong. Looks like a bug.

A few comments on the game if you don't mind.

After white's nonsense opening (losing a lot of time on pointless queen moves) black (you) has a sizeable advantage. Look at the position after 4. d3: you have 2 minor pieces developed (white has 0) plus it is your move. The way to exploit this is to open up the center and attack. Either play d5 immediately, or develop the f8 bishop (Be7 or Bc5), castle and then play d5.

Instead you played Nd4 which loses time and just helps white. Of course this move has some value at low level games (white may easily lose the a1 rook or even the queen) but this is not the way forward.

In the next part of the game, you just trade, trade, trade, until you reach a boring equal position. This is not good. Try to play more active and more agressive chess.

White's last move, Bg5 (instead of Bxc1) was the critical mistake and reason enough to resign. Before that, it was worth playing further.

Why is this Qh5 move so popular today?

After something like Rxf1+ Kxf1 Bxg5 or even Bxg5 first and White is down (rook vs pawn) in material immediately. The queenside pawns have no help. A queen blunder by Black would White's only hope to win

It as black that did Bg5. And white resigned!  White's resignation is a mystery, as it seemed fairly even to me, and the analysis shows white ahead.

Look at it on a board...I gave 2 exchanges where White loses material

Note to OP: Sam Shankland has a book on passed pawns...Small Steps (something, Google it)

Why does the engine put white way ahead then?  the analysis bar doesn't seem to have numbers anymore/currently, but it is mostly white indicating a big advantage for white.

You may have to post a screenshot to show, but I wouldn't rely on the online engine too much.

it has numbers, you need to hover

Apparently the game was won by resignation. There are at least two possible reasons why your opponent could have resigned, even though that position looks very playable on both sides with White having better chances to win.

1. He had thought that the rook was pinned and the bishop could not be moved out of the way at all, forgetting that he had the option of capturing the bishop directly or even leaving it there. This happened to me before where I stacked pieces to attack a pinned queen, forgetting that the queen can capture my pinning bishop directly after I removed the rook (protecting that bishop previously) to gang up against the pinned queen.

2. He clicked on the resign button by accident. This is possibly likely if he had intended to offer a draw.

I looked at the position after Black played Rxc1 (thinking White had played Bg5 next not Black and resigned). Sorry about my stupidity

If play continued...

Bxg5 Qxg5 he was probably concerned about back rank weakness and the rook being attacked. Protect the rook with Qd1 and he loses the passed pawn to Qxd5 (a protected passed pawn). It's pretty much equal at that point. So, maybe he tried to offer a draw, who knows?

White resigned in a winning position. Maybe there was some real life issue that caused him to resign.

don't know