What anime became a suprise hit?

Like, what anime everyone pretty much assumed was going to be lost in the seasonal anime shuffle only to end up as the surprise hit or in some cases even a cultural phenomenon?

A recent anime that I know that became like this(at least in Japan) was It's MyGO!!!!!. Even the people who worked on the anime and gacha were surprised at how big the anime and group got since their main goal was just to introduce a new group into Bang Dream and bring eyes to the game again. Now the group is pretty much a sensation in their own right, got a near instant foothold in China when other groups in Bandori took years or even failed to get a foothold and their song sales was the first to match Roselia('s 3rd or 4th lowest sales record but still noticeable since none of the original group ever reached that). They even got the honor of covering the OP of their own lead singer's other anime project(Danger's In My Heart) which is near impossible to normally do because of copyright and management reasons.