Moments where its almost heart warming, almost touching, almost "Awww" worthy, but something just ruins it, like an interruption from someone, something bad happening, or the characters themselves contradicting themselves or doing/saying something to ruin the mood.
Other forms of this include,
1. The intention was touching, very nice, thoughtful, loving, etc., but it didn't turn out as expected.
2. or the opposite, when what a character does seems caring or touching, but their intentions are less than noble
The equivalent Pixiv tag is 'ｲｲﾊﾅｼﾀﾞｯﾀﾉﾆﾅｰ'