to roll in; to tumble into; to visit (esp. to stay overnight as an unwanted guest); to come to live with
to fall in one's way; to fall into one's lap
友人が突然転がり込んできた。
A friend suddenly showed up to stay.
友人が突然うちに転がり込んでいる。
A friend has suddenly crashed at my place.
ボールが部屋に転がり込んだ。
The ball rolled into the room.