And it wasn’t just defending; he missed a chance to take a winning position.
I think it was the point where he could have won a pawn, but chose to maintain the tension for one more move, and the simplification would actually have been winning.
Also, according to analysis Carlsen blundered before the first time control, which is pretty noteworthy as in most games he registers at most a single inaccuracy, let alone mistake, let alone blunder.
The final endgame was a technical draw according to table bases, but an incredibly complex one for a human to defend on the board. But it’s yet another reason Ian may not feel good about this match (sadly – he played amazingly).