Tianlong-3, a two-stage kerosene-liquid oxygen rocket comparable to SpaceX's Falcon 9, experienced the failure during a test of its nine engines intended to power the first stage.
At 3:43 pm, the engines were fired according to plan, and the engine thrust reached 820 tons. However, a structural failure caused the rocket to detach from its launch pad and blast off.
All villagers in the area, which is remote and far from the city center, had been evacuated before the test.
https://www.youtube.com/watch?v=N8sXEYMw3pA
That is a really bizarre structural failure of the test rig. I'm been a space nerd for my entire life and I've never seen footage like this. For this rocket to lift off vertically, all the mechanisms holding it down had to have failed simultaneously. If they failed at different times it should have gone wildly off-course from the start. That simultaneous failure is statistically unlikely to say the least. I'm wondering if it wasn't mechanical failure, but a failure of the control system managing the hold-down mechanisms.
For comparison, here's what a Falcon 9 static fire looks like. They don't just use the normal launch clamps at the base of the rocket, they also have a heavy cap on top (the orange part) with a cable rig system anchoring it down to make damn sure it doesn't go anywhere. They run all systems of a production rocket at the full thrust and full duration of an actual launch to test everything as a unit.
Further update: the last known incident of a static fire test accidentally launching was in 1952 with the Viking 8 suborbital rocket (no relation to the Viking space probe series).
https://youtu.be/z5rRZdiu1UE