Quite a few, but especially when Taiga finally realizes her feelings for Ryuuji at the end of Episode 19 and tries to run after him, only to find him gone and she starts crying her eyes out. To make matters even worse was the fact that this scene immediately followed a Crowning Moment of Heartwarming, making for a serious case of Mood Whiplash.
Who DIDN'T tear up when they heard: "RYUUJI IS MINE!" Is it possible to have a Crowning Moment of Awkward?
"If it's possible, may Aisaka's father please come up and say something?" *sob*
It's even worse when you consider that right before the beauty show started, Taiga had decided to keep that introduction even though it looked like her father might not be coming, putting all of her trust in him. She wasn't doing it for herself, she was doing it for Ryuuji.
The last minute. The freaking last minute. Some people get angry because they didn't watch through all the credits so it's a downer ending until the last freaking 60 seconds which is the most heartwarming thing ever made in ever.
When Ryuuji and Yasuko got in a fight was pretty damn sad.
All the scenes with Yasuko in the last couple of episodes after she confronts Ryuuji.
Really every episode after Episode 19, it gets sad at the end.
The clapping scene.
When Minori roots for Ryuuji to chase after Taiga.