I had a roll away a couple of years back.
Hooked up to trailer in yard, connected lines, knocked trailer brake off, and the whole unit started to roll forward.
Pretty flat yard but it gathered speed pretty quickly.
Without thinking I ran in front of truck around to driver door and managed to jump in and put handbrake on.
Very dangerous looking back.
This place ran you ragged with 15s and reduced, and I put it down to exhaustion.
I no longer work at places like that but I double/triple check the handbrake every time I get out now